@douwe core concept: accounts are hosted separately from apps, so one account logs you in everywhere.
All data is stored in your account, and different apps can re-use types of data (called Lexicons).
Here’s a second view of the bookmarks I linked above, in a different app https://margin.at/bmann.ca/collection/3m5u77miiyf2h
Apps can be quite simple, because functions are spread across different parts of the architecture.
Feel free to ask more! My atproto account is bridged here @bmann.ca