What is the best way to develop a cross-platform native app in 2021?

Run As Fast As You Can As Far As You Can from frameworks like flutter.

If I were you, I'd develop web app and embed it via web view in native Android/iOS. If you really need platform specific L&F, you can consider react-native.

I'd pick Flutter. It is the one I've been most impressed with and seems to have the best balance of capabilities and trade-offs right now IMO.

If you're going to need a Web App no matter what, I think you should just go Web-App only.