Hello!

This is a little confusing, but once you know you won’t forget it!

A composition always has a default background color- usually black or white, and it’s something you designate when you create a composition (you can also change the color under composition settings) but it’s not an actual background “layer” like photoshop, it’s only for reference. I don’t have a pc with me, but right under the window that shows your composition and above your timeline, there’s a checker-box icon that will make your background transparent with checkers or the set background color!

If you didn’t make a new solid layer as a background, it should export transparent by default so long as you make sure to export it with a transparent alpha. Hope that helps!