Comment by nostrademons

YouTube did rewrite on Google infrastructure - there was a paper they published c. 2008 about how they were leveraging Google's cloud to scale. It was largely back-end stuff; IIRC they moved from MySQL to BigTable, adopted Google's blobstore & CDN, integrated with Google's machine-learning systems, etc. They were allowed to choose which parts of Google's infrastructure they wanted, though. AFAIK the front-end was still Python when I left Google the first time in 2014, and it wouldn't surprise me if it still is now.