I have a ASUS Zenbook 14 OLED UX3405 and it's the same here - thermals are shit. I can't watch a yt video without the fan spinning up. I can in mplayer though, so there must be something in the desktop tech stack that prevents the cores from sleeping. Maybe it's Wayland which I've noticed is sluggy sometimes or maybe Linux tcp/ip handling of video streams isn't optimized for energy-efficiency. The stack is so deep that finding the culprit is probably impossible.