ytdlp.online

How about Catbox?

Linux Distribution (Distro) and Desktop Environment (DE). Not sure why the commenter above expected you to use Linux though

Okay MkvToolnix isn’t the right tool for that afaik, but ffmpeg is and it sucks there isn’t a GUI for it that’s as powerful as the cli which is what I use, luckily you can find a lot of help online,

Apart from that I’m afraid i can’t help much, but good luck with your search!

Are you trying to concat streams or just to remux? For remuxing as roawre said there’s also MkvToolNix, which works great (and it has a gui don’t worry)

ffmpeg, if you need a gui use handbreak

Here you go