Google own vast network infrastructure. The day Google acquired YouTube (I was there) they discovered that YT was on the verge of collapse and would run out of bandwidth entirely within months. There was an emergency programme put in place with hundreds of engineers reallocated to an effort to avoid site collapse, with the clever/amusing name of BandAid.

BandAid was a success. YouTube's history might look very different if it wasn't. It consisted of a massive crash buildout of a global CDN, something that Google historically hadn't had (CDNs aren't useful if everything you serve is dynamically generated).

One reason BandAid could happen was that Google had a large and sophisticated NetOps operation already in place, which was already acquiring long haul unused fibre wavelengths at scale for the purposes of moving the web search index about. So CDN nodes didn't need to be far from a Google backbone POP, and at that point moving bits around on that backbone was to some extent "free" because the bulk of the cost was in the fibre rental and the rackspace+equipment on either end. Also it was being paid for already by the needs of search+ads.

Over time Google steadily moved more stuff over to their infrastructure and off YouTube's own, but it was all driven by whatever would break next.

Then you have all the costs that YouTube has that basic video sites don't. ContentID alone had costs that would break the back of nearly any other company but was made effectively mandatory by the copyright lawsuits against YouTube early on, which Google won but (according to internal legal analysis at least) that was largely due to the enormous good-faith and successful effort demonstrated by ContentID. And then of course you need a global ad sales team, a ton of work on ad targeting, etc.

This story explains why Google can afford to do YouTube and you can't. The reality is that whilst they certainly have some sort of internal number for what YouTube costs, all such figures are inevitably kind of fantastical because so much infrastructure is shared and cross-subsidised by other businesses. You can't just magic up a global team of network engineers and fibre contracts in a few months, which is what YouTube needed in order to survive (and one of the main reasons they were forced to sell). No matter what price you come up with that in internal booking it will always be kinda dubious because such things aren't sold on the market.