What do I need TRWP for?
What does this mean? Is this to make a backup of the entire device in case anything goes wrong?
TWRP is a custom recovery which allows making system backup. You might want to create backups in the process when removing extra apps, you may not like the end result or remove wrong app by mistake.
Restarting after recovery flashing with ESC key held takes me into a boot loop
Try reinstalling it in TWRP app, maybe something went wrong. Do you have X27, X25 or WiFi only model?
I seem not to have MTKlogger and Network Location on my device. I did the whole rooting thing already earlier, though. Maybe I already removed it? Can't remember. Where exactly would I find those Apps?
These are system apps, check System -> Apps -> System for them.
What exactly do I need this for? Is this for determining my location with GSM and WiFi (like GPS), or is it to locate available GSM and WiFi networks? What if I skip this step?
You might want it to have working location functionality. These backends are open-source and don't spy on you; partricularly, GSM&WiFi apps use GSM and WiFi data in a manner Google does, but keep local databases on your device, without reporting anywhere.
Maybe the most stupid question: Why is there an asterisk behind "Google"?
Google * = every app named Google-something.
And one general question: do apps - those I installed through the 'Playstore' (I allways wondered how serious or actually childish an OS is the software repository of which has such a stupid name) and those I download as an Apk file - rely on the presence of "Google services", "Google Playstore", "Google account manager" or any other Google stuff? Do I have to expect apps to stop working once I remove all "Google" parts? That would be especially annoying if it concerns apps I payed for, but of course all other apps as well. If I wouldn't miss it, I wouldn't have installed it in the first place.
I would not recommend downloading APKs directly unless you're pirating something from a very trusted source - there's Yalp store for that. Some apps report that they're relying on GSF (Google Services Framework) but most or all functions work correctly without it, while others will misbehave.
Thank you and kind regards
You're welcome! Hope you'll achieve your goals!
p.s. I personally am currently experimenting with keeping the google apps installed (most of them disabled) and routing Google traffic through interface directly while routing browser and messenger traffic through VPN, with properly setup browser of course - because there is no way to make voice recognition work without some Google stuff, even Saiy (FOSS voice assistant) relies on Google engine for speech recognition, sadly there's simply no working alternative.