These are per-connection bottlenecks, largely due to implementation choices in the Linux network stack. Even with vanilla Linux networking, vertical scale can get the aggregate bandwidth as high as you want if you don’t need 10G per connection (which YouTube doesn’t), as long as you have enough CPU cores and NIC queues.

Another thing to consider: Google’s load balancers are all bespoke SDN and they almost certainly speak HTTP1/2 between the load balancers and the application servers. So Linux network stack constraints are probably not relevant for the YouTube frontend serving HTTP3 at all.