Hello, is compose multiplatform on iOS is pure native just like building UI with swift UI?,

It's canvas-based. There's an internal ComposeUIView, and Compose draws to that with skia. If you're looking for just something that binds to UIView, Cashapp is working on a library called redwood for that.

It depends on how you define "native". There was an interesting thread about it recently: https://twitter.com/JimSproch/status/1646483451212877824

I wonder, then why does it use Swift? only iOS use Swift in kmm.

What do you mean "why does it use Swift"? Swift is used to setup Compose (and you may add as much Swift as you'd like).