And yet, problems were constantly found and fixed and improved.
No two falcons were ever built exactly the same until Block 5 or so (and honestly, I'd be surprised if there weren't changes between rockets coming off the line now, but the changes are much smaller and more iterative, I would guess).
It's basically the software development process, just with hardware. Falcon 9 is a mature product at this point, so most of what's happening are maintenance changes.
No two falcons were ever built exactly the same until Block 5 or so (and honestly, I'd be surprised if there weren't changes between rockets coming off the line now, but the changes are much smaller and more iterative, I would guess).
It's basically the software development process, just with hardware. Falcon 9 is a mature product at this point, so most of what's happening are maintenance changes.