Combing video layers/export transparent gif

Hi everyone I am making some websites and getting Elements subscription and see videos that have black background and some diagonal line half black background half what I think is transparent background and want to easily combine these video clips into other video clips and or also export them into a transparent gif file.

My question is why black background why not green as that is standard for most programs and then how easy would it be to export into an animated gif file autoplay loop file so I can put into my website projects.

Thanks for the help!

Hello again,

I am still looking for advice on this topic. I am aware of keyin and keyout on video programs, but wish there was a way to download transparent gif file not just mp4 so it can be used like stickers anywhere on websites like can be found on giphy

Wix just came out with sticker website elements so starting to be popular trend.

As for 3D pictures why can we just download angles we want why not entire 3D picture that we can ebed into our own websites?!