How do I save my Minecraft worlds to the iCloud?

You can't choose to put data from an app into iCloud storage. The app developer must provide that as an option. You can check by going to Settings, tapping on your name at the top, then iCloud. If the app has the capability to save to iCloud then you will see it listed, and you can turn it on. As I'm not sure of your purpose in asking you cannot save an app itself to iCloud, only its data.

Other than backup, as razmee describes, the app you are using must have an option to save to iCloud.

Vadar134 wrote:

Anyone out there in the world who knows how to save a Minecraft worlds to the iCloud!

https://support.apple.com/en-us/HT207428

app data is part of the icloud back up.