QC means you don't sell duds, you test them, if they fail you don't send them for sale.

QA means you make sure that products aren't duds, you do that by finding what causes the failed products to be made and fixing it.

Meta: The focus on QC instead of QA brings to mind the UK government approach to education, adding more tests instead of using the time/resources devoted to tests to educate the children.

You need both; they serve different functions. QA is choosing to use flame-retardant material; QC is installing a fire alarm.

QA addresses your process, from repeatability (e.g. ISO9000 in the cases where it makes sense -- less often than it is used, but sometimes it's crucial), incoming materials inspection, choosing processes that error in directions you want (should you mill material away from a billet, potentially taking not enough, or cast and potentially have the part shrink? Some systems can handle a too-small part better than a too-big one; other systems are the opposite) etc.

QC can be thought of output validation (i.e. testing, which is a lot of it): are my products of the necessary quality/within spec etc? Sometimes you can do 100% inspection, sometimes you have to use statistical methods (there are whole ISO specs devoted to nothing but sampling algorithms).

If you are not clear in your mind about the differences, the jargon can confuse you more (e.g. incoming materials inspection is sometimes referred to as "incoming QC"). But from this you can see why you "can't test your way into quality": the tests happen too late in the process.

QA is like linting, having a style guide, having a strong type system and so on.

QC is looking for bugs for in the usual suspects for every output and fixing them as you discover. This can be automated and/or manual.

I am not they, but QA is process oriented and QC is product oriented. How you do it vs. what the results are.

QA is defect prevention. QC is defect detection and removal. If an activity causes rework then it's QC, not QA.

QA is about how you work to ensure you build a quality product QC is about the tests you do to ensure a quality product has been built.

Etymologically, Quality Control implies some sort of feedback. Quality Assurance merely implies a filter at the end of some process.

QA is checking that your output is good.

QC is making sure that your output is good before it gets to QA.