Best editing software for MacBook Air?

Thanks Jack. I figured there had to be a practical solution to this. So I do a backup copy of my current os, in case I screw up and loose everything, then download this Catalina OS that will be able to run iMovie. As you can probably tell, I'm not super knowledgeable about computers and also this is my first Apple and so far I've done quite a bit more screaming at it than I was expecting to. What's up with the invisible scroll bars? they magically appear one minute, then for no reason they disappear the next. That's f'd up!

Is the upgrade free, and where would I look for something like that? By the way, thanks for your help. This stuff really is frustrating, and it looks like you have answered questions for a whole lot of people, so thank you for helping to save so many people's sanity!

It isn't anywhere to be found amongst all the other programs that are listed here. When I tried to download it from Apple store, it said my operating system was too old for it.