The issue:

E.g. Subsite A has List A; Subsite B has List B

I want to show a list on different Subsites in the same collection.
Technically establish a CRUD like connection/two-directional sync, so I can Create, Read, Update and Delete on either List (or webpart thereof) and have it update the other list (or a central one).

I tried webparts, but 'Apps' of other subsites don't show up.

We have on-premise Sharepoint 2016.

Sorry if my definitions are not exactly correct, but I'm translating from German where there are Websites and Websitesites - a terminology mess.

I am fairly new to Sharepoint - but I'm not bad at IT stuff. So if you can at least point me in the direction of what to search for, because I'm close to pulling my hair out, because I just don't know how to find the knowledge I need.

Good day.