Clarification: You can have multiple connections (aka file descriptors) open on the same database file for writing at the same time. But only one can be actively writing at a time. SQLite uses file locking to serialize writes. Multiple writers can exist concurrently, they merely have to take turns writing.

WAL allows multiple readers and writers:

https://www.sqlite.org/isolation.html

Yes, but in some cases it could be faster to work in sole- writer mode. It is kind of cooperative multitasking demonstrating by Node.js.