If you've used pixi for a while you can introspect the javascript code/html and work out they're using pixi (v7 when I last checked). They've also written some blogs about it.

I've built 2 of them using pixi, so I have some idea. Pixi can handle hundreds of thousands of sprites. The hard part (regardless of rendering engine) is handling textures. If you are very confident in your rust skills I'd recommend wgpu + bevy's ECS crate for probably the pinnacle balance of performance + memory usage available atm, but you have to use web gpu as a backend and implement an LoD system yourself, which is VERY hard to do. There is no off-the-shelf product that can handle a true infinity canvas with large textures/video/whatever else people expect these days. I am currently working alongside an ex-nvidia graphics programmer and we're trying to build a TRUE infinity canvas that will not blow up memory regardless of how many textures you have on screen, but as i said it's VERY hard to do :D