It's actually quite simple. Don't use the Java edition at all. Just use the bedrock edition which is crossplatform with Windows 10, consoles, and mobile devices.

You would need to code the whole minecraaft networking again on serverside btw vv

It COULD be possible, but you would need to code tons of code and handle player movement map gen etc on serverside.

Example would be that you have same map on both platforms, when player moves it moves clone on another platform´; when player breaks/places it would make the clone to place or break. But there is no way to make it easily and clientsided ;D

Where Java edition and Bedrock edition have completely different code and some features and details exclusive to their versions, I am not aware of a way Java edition players can play on a Bedrock server and vise versa. You can play cross-platform between XBox and PC if using the Microsoft edition of Minecraft, but I don't believe Java and Bedrock will ever mix.
Anyone can correct me if I'm wrong, but as far as I'm aware, it's not a feasible thing.