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You can opt out, but I think it will be the default.

Not required. You can choose between iCloud backup, iTunes backup (even encrypt if you want) and no backup at all. iCloud is the most convenient though

Just have more than 5 GB of data and you can't backup to iCloud without paying monthly.

Photos are not included, Apps aren't (only their data), I'm not sure about music. It's gonna be hard to exceed the 5GB this way

Photos may be included.

"Here’s what iCloud backs up: Photos and videos on your iOS devices, unless you turn on iCloud Photo Library on your iOS device (iOS 8.1 or later)."

https://support.apple.com/kb/PH12519?locale=en_US

The photos hit the same 5 GB limit, so it's not like excluding or including them really changes anything. Photos are the #1 reason people hit the backup limit.

Thanks, didn't know that (I use iCloud Photo Library), but IIRC you can disable stuff from backing up, even photos

I currently use 18 GB with only a single iPhone. 14.6 GB of this is my iCloud Photo Library.