You basically just have to assume that they are doing crap in the background unless you get one of those de-googled custom roms that runs google play crap in a sandbox.

I gained like 10% battery per day by uninstalling a few of them.

telling if they’re doing stuff

I think foss apps like Netguard and Trackercontrol would be useful to use if they connect to the net.

Restrict them

The same apps can be used to restrict connections.

Android app preferences can be used to restrict background data and background battery usage too.

And if you want to remove unwanted apps that can’t be directly uninstalled, you can use the Android debloater.

I wish I knew, I gave up on figuring it out and decided it would be easier to just delete all the apps and only reinstall them briefly when I need them.