Here are two of my favorite video editing apps.
Hitfilm - Free video editing software for Mac and Windows. It has two main functions. You can line clips up along a time line (editing) and/or you can layer videos on top of each other (compositing). They also offer a professional version of Hitfilm ($200-300) that has many more features. I've been using the free version for years now, and it does everything I need and more. They have a
very helpful forum.
CrazyTalk - This fun software allows you to animate photos of faces in an endless number of ways. You can also make your characters speak, by typing in text, or importing an audio file. Currently selling for $99, but you can likely get it cheaper if you get on their mailing list and wait for a sale. They have a free trial version so you can test before purchase. Many quality video tutorials on YouTube.
These two programs together can be very useful. Here's an example use to illustrate.
You could for example, animate a photo of former President Dumpster in CrazyTalk, and have him say all kinds of ridiculous things, though it may be hard to think of anything more ridiculous than what he actually says. Then, using Hitfilm you could layer your talking head of President Dumpster on to a video of a pig so as to replace the real head of the pig with President Dumpster's talking head.
Or, if you're more mature than I am, you could use these products to do actual useful things. I've heard that can happen.