Find what is broken before the client does, and write it down clearly enough to fix.
We do not have a QA layer today, which is exactly why this exists. You would build the checklist that every project goes through before a handover: the flows, the edge cases, the payment paths, the phone that only has 2 GB of RAM.
A good bug report here saves an afternoon of arguing. A vague one costs the same afternoon.
The job also means being the person willing to say a release is not ready.
Manual testing across real devices, writing reproducible bug reports, API testing. Automation experience is a plus.