I am playing MC on an Android where I have a Realms account with a world in it.
Is it possible to copy this world on my iPad, so that I can play locally (not Realms)?
If that world is still on the Realm (doesn't have to be active), then yes! Make sure you sign in with the same account on the iPad that you use on Android. Then click the pencil next to the Realm, scroll down on the right side of the screen, and click export world (maybe download? They change the button depending on the device). It will now be a local world on your iPad
As a side note, if the Realm is still active, you can transfer saves between any device running the Minecraft Bedrock Edition (iOS/iPadOS, Android, FireOS/FireTV, Xbox One/Series, PS4/5, and Windows 10). You can also use the Realm to sync your data between devices if you plan on often playing on both and don't want to bother with transferring every time (this also means you can invite friends to play, even if you are offline).