I want to play minecraft with a friend of mine but I'm not sure of the specifics of the better together update and realms. I play on Xbox one usually and they play on Ipad. Would I need to purchase the realms subscription in order for us to play together or could I just start a world?
If you can already play together, then you don't need to purchase Minecraft Realms, but it depends on what kind of world you want. If you can't already play together, then you will only be able to play together with the Windows 10 version.
The Better Together update just made all the versions except the one on PC/Mac cross-compatible by turning them all into Bedrock Edition. Minecraft Realms is just a server service run by Mojang so that people don't need to host their own server. Realms is only necessary if you want your friend to be able to access the world when you aren't online.