Is it possible to transfer minecraft worlds across devices?

I've actually done this a few times - taken a copy of a world from my Xbox One so that I could play it on my Windows 10 PC (the concept is the same) - but there are a couple of prerequisites:

1. You'll need an active subscription, whether paid or free trial (see bullet points), to Minecraft Realms/Realms Plus.
   • Realms/Realms Plus is basically a private, invite-only Minecraft server hosted by Mojang/Microsoft.
   • If you've never availed yourself of the free 30-day Realms trial offered by Mojang, you can use that to complete the upgrade/migration process no additional cost. Simply register for your trial and then cancel the subscription before the end of the 30 days to avoid being charged. During that 30 days, you'll be able to follow the instructions below as many times as you want/are able to migrate several worlds for free.
2. Your original world must be from *Minecraft Bedrock Edition *
   • "Minecraft Xbox One Edition" is not the same thing as Bedrock Edition. The latter (on the Xbox One) is simply called "Minecraft" without any qualifiers.
   • If you're world is from Xbox One Edition, you'll need to migrate it to Bedrock Edition before it can be played cross-platform (see below if this is the case).
3. You can only have one of your worlds active on your realm at a time (unless you pay for more than one subscription).

Using a pre-existing world on your realm

Once you have a Realms/Realms Plus subscription, you can choose any of your worlds from your Xbox One (Bedrock Edition) to use on the Realm by doing the following:

1. Launch Minecraft (Bedrock Edition) on your Xbox One and click "Play".
2. You'll see a section at the top for "Realms", and your realm from your subscription should appear there. If you don't yet have a Realms/Realms Plus subscription, (I'm pretty sure) you will have the option to create one in step #5 below.

3. Find the world you want to be able to play cross-platform (on your Switch, Windows 10, Pocket Edition, etc.) and click the pencil icon to the right of that world. You should see the following:

4. Click the "Play on Realm" button.
5. You'll be prompted to select a Realm Server. If you've already got a Realms/Realms Plus subscription, simply select your realm from the top of the list. If you don't, (I'm pretty sure) you should be able to "add" a subscription from here by clicking the "Add a [X] player Realm" button of your choice.

6. You'll be prompted to confirm the replacement of any existing world that has been created for your realm before continuing. Simply click Confirm, and the necessary files will be uploaded to your realm to make available for anyone to whom you've given access.

Now, at this point, you can do a couple of different things. You can either leave this world on your realm so that it can be played by anyone you've invited at any time. All changes from any players will be persistent and show up for everyone else anytime they log in. A realm is, by definition, invite-only, so only those people to whom you've given explicit permission to join will be able to access it, so you don't necessarily have to worry about random "griefers".

Your other option is to download the world to your new device to play it locally. Of course, this means that any changes you make in the local copy will need to be re-uploaded if you want them available on other devices. However, this can be a useful tool for creating copies of your realm world for testing out certain builds and such (if that's something you're in to).

To download the world from your realm for local play, simply click the pencil icon next to your realm on the world selection screen and you'll see an option to "Download World". The game will process the game information and make a local copy of the realm world, which will show up in your main worlds list.

Migrating a world from Xbox One Edition to Bedrock Edition

If you are wanting to play an older world from the Xbox One Edition of Minecraft on your realm, you'll need to follow these additional steps to migrate/convert that world to the Bedrock Edition format:

1. Launch Minecraft (Bedrock Edition) on your Xbox One and click "Play".
2. Scroll down to the bottom of the worlds list and select "Sync Old Worlds". This will add a section below the main worlds list titled "Old Worlds", which should contain any worlds created in the console-specific version of the game.
3. Find the world you want to convert to a Bedrock Edition world and select it. Minecraft will run through some processes to complete the conversion and add it to the top of your main worlds list.
4. Scroll back up to the top and select the "new" world to play it on Bedrock Edition.

Final Thoughts

If you're starting with a world from an even "older" generation (e.g., Xbox 360 Edition), it becomes a bit more complicated as those worlds cannot be directly migrated to Bedrock Edition. Worlds from the Xbox 360 Edition will first need to be converted to the Xbox One Edition before they can be moved to Bedrock Edition.

More information on migrating worlds from various console-specific editions to Bedrock Edition can be found in the official Minecraft Help Center. See the section titled Minecraft on Consoles for details.

Lastly, just for the sake of completeness, for anyone who's wanting to copy a Bedrock Edition world from one PC to another, you can simply transfer the MCWORLD file between the two computers using USB, DropBox, Google Drive, etc. You'll need to copy/export your world, then transfer the MCWORLD to your storage medium for retrieval on your other device.

RELATED

I found another, similar question here (How to share worlds between platforms with Minecraft Better Together) that offers some possible alternatives to using Realms, but I've not tried these other methods and can't speak to their functionality or reliability.

The previous answers do provide one possible solution, however, if you are on a mobile device or a computer, you can also access the world files directly, copy them to a media transfer device, and transfer them that way. This is similar to the way uploading/downloading maps works for adventure and puzzle maps (which is another option you have to transfer your worlds). I'm not sure if either of these methods work for xbox, switch, or playstation, as I've not attempted to transfer any worlds that way.

This is especially useful if you play a world with a friend, and your friend wants to have a copy of the world for themself, or if you wish to backup a world in case it gets deleted or otherwise lost.