It’s been a long time (my kids are 8 and 10) so I’m sure features have changed but I figured I’d chime in with my favorite baby monitor feature. Not all of them had it but I found it invaluable.
I think ours were Samsungs? And they had a “vox” feature. Essentially, if the room was quiet, the monitor screen and microphone would turn off. If there was a noise, it would pop back on. (In this way, it’s different than an audio-only monitor which I presume is on all the time.)This allowed me to sleep (when sleep was available…rarely with our first) and be awoken when needed. I was way too sleep deprived for Nest Cam alerts on my phone to actually get through to me, especially since they popped up once; if I missed it then, it wasn’t going to keep nudging me.
That said, the monitors themselves were cheaply manufactured junk, and not inexpensive. I liked the vox feature enough that when the first one started having problems with the antenna, we bought a second one and immediately taped the antenna down. Hopefully you have better options out there these days.