I have an M1 Air, so perhaps the M3 get warmer, but I do not notice my Air getting warm when watching videos. I just tried watching a 4K video and still didn't notice it getting warm, even specifically looking for it. As a test, I just did a Handbrake MP4 conversion of an EyeTV recording I had made of SNL, and at that point the bottom did get warm, as it took an hour and a half to convert the video because it still had commercials and was converting from my NAS at 31 FPS (the highest CPU usage spike I noticed in Activity Monitor for HandBrake was 641%, as it is great for multicore usage). It was warm enough to feel, but it wasn't so warm as to be uncomfortable for bare legs. I also watched part of that earlier 4K test video while encoding this with no issue. I had/have 26 Chrome windows open (13 open/13 minimized) during the test, most with around 10 tabs each, though the window with this MacRumors tab has 39 tabs open. Loading a new MacRumors tab in Chrome with the encode running was a bit laggy for loading the article images, though. I didn't close Mail, Messages, Calculator, Remote Desktop, or any other app to test.
So maybe I got lucky for once in my life and somehow got the best example of the M1 Air 8GB/256GB made and I should actually be having memory and heat issues that some mention, but in my experience the MacBook Air became a truly excellent computer with the transition to Apple Silicon.