Unfinished work is not typically checked into master (and it's certainly not regularly broken).

What is more common is that very large changes are checked in as a series of individually compatible changes, and often broken up across the repository (there are of course tools to help with this). It's relatively rare for multiple developers to work on a single changelist; it's much more common to break the work into separate changelists.

Haven't worked there for some years now so I'm a bit rusty on some of the detail.