Browser extensions are very similar, just as an extension instead of direct. Unfortunately, it's difficult to use non-store extensions in Chrome, and Google pretty much blocks anything that allows youtube downloads from the store. youtube-dl is likely the most popular option for this, and even then, the same (deeply flawed) logic could be used against any of them.

> I wonder how many of those use YouTube-dl on the backend

i wager to say that the vast majority of browser extensions and websites that allow downloading of media content use ytdl in the background