I know of:

• LineageOS
• LineageOS+microg (is its own thing)
• GrapheneOS - this is what I use
• eOS - shiny degoogled Android
• iodeOS - degoogled Android with some extra features and parental controls - might be something you want?

Some other Android-derived projects are inactive: DivestOS shut down, CalyxOS is paused.

For 3, I think it has to do with popularity, unlockability, and the chipset.