@ Creators 또는 just computer geniuses in general: how do 당신 create videos? what software do 당신 use? can it be done online?

That might have been windows movie maker. :D It isn`t in the newest win OS.
Mostly normal quality video editing software needs to be paid for, so, if 당신 are going to do it professionaly, 당신 can buy something like Adobe Premiere Pro CC 또는 Corel VS. If 당신 are searching for free software to use just for fun, there are several options, but 당신 have to go and check out what each of them offer to find what fits your needs the best. :)
There are also online editors, but they don`t really have many features 또는 are pay-to-use sites.

I always used windows movie make but windows no longer has that...

I used it to make nightcore and shitty crack vids. Unfortunately, my last one got taken down for copyright but I don't get it because I used a lot of the same clips from this one and its still up.