I'm not a regular user of Anki. But if I start again, how does it work when someone want to upgrade a package to a more recent version ? Does it merge correctly if I have added some cards or added some personal fields to the "Note Types" (and updated some card consequently) ?

I'm still pretty new myself, but I'm pretty sure adding card works but changing fields doesn't.

This support landed late last year as part of https://github.com/ankitects/anki/releases/tag/23.10

> .apkg imports are able to merge changed notetypes, and can exclude scheduling data