Calculator: Unitto, also functions as a unit, date and time zone convertor. Very useful

Camera: Don't know

Calendar: Fossify. I have tried etar in the past but didin't like it (can't remember why)

Do FOSS camera apps (on Android) support "live pictures" - the type with several frames from which to choose your favorite? I really like that feature in Google's camera app. I do wish I could figure out how to export them as GIFs or something.

Camera: GrapheneOS's camera app. Have been my camera of choice ever since I've discovered it. Never had an issue, offers all the basic features you would expect from a camera

Meta: just making sure that anyone that reads this comment is aware of the existence of Obtainium!

Obtainium allows you to install and update Apps directly from their releases pages, and receive notifications when new releases are made available.

I use num++ from fdroid because it's the only one I found able to represent expressions in "richtext" instead that a plain text line of numbers, because it is much easier to double check for errors

Camera - OpenCamera. Surprised there's been no mention of this so far. OpenCamera offers quite a few pro features like RAW capture, customisable exposure bracketing/HDR, noise reduction etc.

Calculator:

• desktop: Speedcrunch
• mobile: Unitto

Speedcrunch is so nice I never explored alternatives. On mobile, I tried a bunch after posting to Lemmy, and while a lot of them are very similar, this was my favorite.

Camera:

• mobile: I use the built in app, but also keep GrapheneOS Secure Camera installed

Calendar: 🤷