Chat app CSS illustration

Didn't manage to make the bottom-right bg element with ::after and position: absolute on body. It overflows to both bottom and right and I couldn't find a way to fix neither. overflow-x hides horizontal scroll but it's still swipeable on mobile. Bottom overflow was really strange as it overflowed past itself and even html tag.

So I did that second background shape by making an svg file and using it as a background, it worked flawlessly.

If you managed to do it with ::after on body and so it matches screenshots and not overflows, please leave a comment, I'd love to know how it's done.

I'm also open to any feedback regarding html/css. Did I use that aside tag correctly? What would be the best semantic structure here?