Drop in a song and a picture. You get back an MP4 you can upload straight to YouTube.
Nothing is uploaded anywhere. Your song stays on your own computer.
You can hit that as many times as you like. It stays here until you close this tab.
Your browser does the whole job. The song and the picture never leave your computer, so nobody else ever sees them.
The first go takes a bit longer because your browser has to fetch the video engine, about 30MB. After that it is quick.
You get a 1920x1080 MP4. The picture sits still for the whole song. That is the format YouTube wants, so it uploads with no fiddling.
As long as you like. There is no time limit. A three minute song takes about a minute and a half to make. Longer songs take longer, roughly half the length of the song.
The only real limit is the file size. Up to 300MB for the song, which is hours of music, and up to 60MB for the picture.