Warframe

Yeah, understanding how mods interact gets progressively more important as you go. I can explain how to make your weapons better, at least, quickly:

Each 'category' of damage is multiplicative to other categories while all mods within a category are additive to each-other. The categories are Base (ex Serration), Elemental (ex Stormbringer), Multi-Shot (ex Split Chamber), Critical (ex Point Strike & Vital Sense), and Faction (ex Bane of Grineer).

So, you could get the Arsenal to show you high numbers slapping all your elemental mods on a gun but it won't be as effective as using 1-2 of each category. So, make the combo element that hurts whichever faction you're fighting and use the appropriate Bane mod and you should cut through Sedna like you're back on Earth.

But there's more; There's several types of special mods that are only gotten from a few places:

• Nightmare Mods: They give two positive effects. Nightmare mods tend to cost less mod cap and give less of the effect but getting two effects from one mod tends to be worthwhile (so long as both effects are good). They're only acquired from Nightmare Mode Alerts that show up in places you've already unlocked.
• Corrupted Mods: Give a positive effect and a negative effect. The positive effect tends to be larger than you could get on a normal mod when maxxed out and for some weapons or warframes the negative has no or almost no impact (ex less fire rate on a gun with 1 shot magazine, you reload rather than be slowed by fire rate). You get these buy opening the Dragon Vaults on Deimos missions which will require Dragon Keys.
• Galvanized Mods: Give a positive effect and a conditional positive effect (ex more crit chance for a few seconds after getting a headshot) these are extremely powerful mods but you can only get them from the Arbitration shop on relays, which uses Vitus Essence from the Arbitration gamemode as it's currency. You need to do every node on the star chart to unlock it.

You can use these various special mods to squeeze even more power out of your gear, but you'll likely need Forma to make best use of it all. And Orokin Catalysts.

You're also at the point where you can start working on your operator. You want to enter the Plains of Eidolon at night (Set match making to solo to avoid randomly getting put into a public Eidolon/Tridolon hunt), and you can kill Vomvalysts for their cores, and find Cetus Wisps which spawn near pools of water and appear on your map as an item icon. They can also be given as a Cetus Bounty Reward if you don't want to do laps around the map, and you'll get other useful rewards from doing bounties. Probably best to stick to the lower level ones, nothing prevents you from accidently selecting a mission with enemies above your level range.

Find the Quills in Cetus to trade in the cores for standing to rank up their syndicate and purchase the parts for your first amp. Anything is better than the mote amp, but the first amp you will make will probably use the Raplak Prism, Pencha Scaffold and the Clapkra Brace. Most of the resources needed to craft these components are found on the Plains of Eidolon by fishing/mining. Amps crafted this way are modular items and will require you to take the crafted components back to Onkko to assemble them. Amps can be leveled to 30, and then brought back to Onkko to be guilded. This lets you name your amp, colour it, and put arcanes into it (not relevant for your current progression but does become important later on). To change your amp, go to your orbiter* Pause > Equipment > Operator > Equipment, and you should see the option to equipt another amp. *Should also be able to do this from any of the open world hubs, not an option in relays.

If you talk to Konzu at night, you will see the Eidolon Bounties available. At your current level it is probably best to avoid them. The first Eidolon will be doable in a squad but I still think think it's probably too early for you've currently progressed. It is a good source of items to trade for standing with the Quills though.

If you're having trouble finding them, you should be able to fast travel there from the pause menu. You will see a stone door there, use operator mode to enter it. If you don't have the option to fast travel there, you want to start from the gates and hug the left wall and go up a staircase until you find a stone wall, and use operator mode to enter it.

The "I have one warframe" is a problem too. Limbo isn't great either, unless you build it one specific way (and it's buggy iirc). Best to target a specific powerful warframe. Rhino is a great one that's incredibly easy to get. It's extremely tanky and has some crowd control. Saryn is basically a kill aura when you build it right. There are some that favor specific playstyles. The game favors meele weapons and AoE. I'd tell you what I have but I sadly have the game uninstalled right now. Overall, it shouldn't be a problem for you to build an insanely overpowered frame + weapons combo and wreck the game.

You’re probably missing some key mods on both weapons and warframes. I know I made it all the way to Sedna without picking up multi shot mods, which are huge boosts.

Are you in a clan? Your clan should be able to help with some of the beginner rare mods and builds.

Warframe's early game is a bit of a mess balance-wise, and has very little guidence.

Make use of nightwave for some useful Aura mods, and you can get some weapon/warframe slots for ranking it up. Any challenges that aren't completed by the end of the week just get banked and become available again once you've completed some of your current week's challenges. The weekly challenges never really expire until the end of the current nightwave cycle which tends to go on for quite a while. You can purchase Orokin Reactors/Catalysts from nightwave to increase your warframe/weapon mod capacity to 60 (for a rank 30 warframe/weapon). I don't think I would really want to use many of them on a base warframe/weapon, you're better off using them on a Prime instead/or if there's a base warframe/weapon you tend to use often. Once you progress further into the game, you usually have more Reactors/Catalysts than you can use.
Forma is the other way to fit more mods into your warframe/weapons, a little build restricting because they need to match the polarity to reduce the mod cost (or penalty if it's a different symbol), but that's where the min-max fun is if you're into making builds.

If any build guides suggest "Galvanized Mods" or weapon arcanes, then you're looking at a guide aimed more for very late game. Modding generally follows the same pattern for all weapons, Base Damage, Crit Chance, Crit Damage, Multishot, Element, Optional/QOL.

For a primary weapon you want:
Serration, Point Strike, Vital Sense, Split Chamber and your other mod slots are for an elemental combo/option mods (reload speed, fire rate, etc). You might have noticed a planet will suggest an element type like Viral, you would go into the mod screen for your weapon and type in "Viral" into the search box and it will filter mods that can be used to make that element.
Secondary Weapons: Hornet Strike, Pistol Gambit, Target Cracker, Barrel Diffusion, element and optional mods. Melee: Stance Mod, Pressure Point, Organ Shatter, True Steel, element and if your melee weapon is slow then you will want a mod like Fury on to increase the attack speed, otherwise this mod falls under the optional category. If you have the Skijati, that weapon comes with the Sacrificial versions of point strike/organ shatter along with the polarities to use those mods, so use those instead of their base versions.

One last thing is if you haven't joined a syndicate yet, pick one and rank it up. Either pick New Loka, Perin Sequence and Red Veil OR Cephalon Suda, Arbiters of Hexis and Steel Meridian. Someone probably can probably do a better explanation on how these syndicates work than I have time to write right now/there should be existing guides which should be able to explain how they work in a much less confusing way than I would explain it.

The Meta changes fairly often, but most of the builds you can find on Overframe are still more than viable as most of the top rated bulds are made for steelpath. You can always take a bit of inspiration from there. Just remember to read how it works.

Could you post your favourite loadout, in terms of the frame you use, the weapons you use, and their mod configs? I can give more tailored advice with that information.

My suggestion would be to watch a new account/F2P playthrough on YouTube and learn from that - I know Brozime did one fairly recently. Iflynn had a pretty great beginner guide too, but it's slightly older, 2022 I think?

The answer is basically mods though - you need to learn how to mod both frames and weapons, and it can take a while (and can seem super complicated and daunting at first). The free to play playthroughs are a great starting point, since they show it step by step, without spending real money or platinum.