>>29260693
ffmpeg
I'm not going to teach you how to use it because it really boils down to scenarios.
It can do many things. Take audio from video files, slice video cuts, take a folder of images and turn them into a video, then add audio into it. It does a great number of things and you're going to have to google everything you want it to do, but in your case you'd be googling "ffmpeg output webm, quality vs length" or something like that. or "compression types"
It's an absolute rabbithole and the original devs deserve a goddamn nobel peace price for their robustness.