I was working at Google when this was still going down. Pretty much what happened was YouTube came and had a little party at the Googleplex and said "Hey guys, we're growing like crazy and have tons of crazy scalability problems that Googlers love... who's game for helping out?" and if you were a Googler that was interested... you just switched roles over to Youtube. Youtube more or less just trudged along and grabbed the talent/resources from Google as they needed them.

It was kind of the ideal situation for a start up... you have no monetary concerns, as soon as you need talent you can go pick from the brightest engineers in the world and have them good to go the next day, and on top of that you effectively have unlimited computing resources... which is important when you're doubling at the rate that Youtube was.