YouTube buffering is happening "out there on the internet" or between the your ISP and you. I just now ran a speedtest and, while it's slower than my desktop going to the router over Ethernet, it's still a healthy 86.6mbps, more than 10 times the speed you need for anything from YouTube. (Pixel 2 updates, but that shouldn't make any difference as large as 86.6 to buffering.)