There's definitely testing but it's not at the level that it should be. That is, there's presubmit and postsubmit testing, but who is triaging the results? In addition, how often do we see people design tests to pass instead of design tests to fail and catch issues? I can't speak for Youtube, but some regression can be perhaps attributed to what management claims is lower productivity for new grads and mid levels, who are often assigned things like writing tests and automation (everyone in every company just wants to work on features). I think manual QA is highly underrated in general in the industry and catches so many bugs that automation just can't, and my guess is the manual QA efforts has decreased, and the discrepancy in output from seniors and juniors/mid-levels might be causing some quality issues (just my guess).