You do everything the same as today. Then you turn it over to QA who keep finding weird things that you never thought of. QA finds more than half your written bugs (of course I don't write a bug everytime a unit test fails when doing TDD, but sometimes I find a bug in code I wrote a few weeks ago and I write that up so I can focus on the story I'm doing today and not forget about the bug)

QA should not be replacing anything a developer does, it should be a supplement because you can't think of everything.

We also use QA because we are making multi-million dollar embedded machines. One QA can put the code of 10 different developers on the machine and verify it works as well in the real world as it does in software simulation.

The are breakers. Good devs are makers.

You can be both but I have yet to meet someone who is equally good in both mindsets.

They find all the things the devs and their automated tests missed, then they mentor the devs in how to test for these and they work out how the bug could have been found earlier. Rinse and repeat until the tester is struggling to find issues and has coached the devs out of his job