I like to call this a "macroservice" or "business function" repository. If you are dealing with the Billing function, everything to do with that is in the billing repository (UI, API, scripts, workers/jobs, database migrations).
This is my favorite for sure in terms of balance - you don't need the overhead of one repo per deployable artifact, and the size of the repository has a reasonable theoretical maximum and can still come close to fitting in your head.
You can also test it all together extremely easily and it's the perfect slice for a team to work on, or be shoveled off to a different owner of that function in the future.