How to share worlds between platforms with Minecraft Better Together

There is a way to transfer worlds from the Xbox to windows 10 and back using Minecraft Realms, but it will cost some money and availability may depend on your country.

You sign up for the Realms service in the game (on either platform) and then the realm appears on the list of worlds you can log into. A pencil button next to the realm name opens the Edit dialog, and that screen has two buttons you will use. First use "Replace World" which allows you to upload a world file from your local gaming platform to the realm. You'll do this on the Xbox, or the one that has the world you're trying to transfer. Then, on the other platform, used the "Download World" button to import the world.

I've just tried this and it works fine, but it does require a subscription to the Minecraft Realm service. I think you can sign up for this service through multiple different stores, like Google Play or the Microsoft Store. Tonight it was $3.99 / month for the "You + 2" monthly subscription, I'm sure that will eventually change.

You can cancel after transferring the world if you want but if you plan to play on both platforms you will probably want to just leave it subscribed, then you don't even have to mess with downloading the realm. You could still download the world as a backup, but the service will also create backups periodically. These appear in the in-game menu and the world can be restored from them, but I'm not sure what schedule they are created on and there doesn't seem to be a way make it take a backup.