Do you have to use i cloud when you have a iPhone ?

No, but you lose a lot of features if you don’t use at least basic iCloud services. You do need an Apple ID, which becomes your iCloud ID. But you don’t have to use it for iCloud.

The fact that you logged in to this forum means you have an Apple ID.

No you don't *have to* to use an iPhone. But you will need it for some features, like sharing.

If you want to sync between other Apple devices or do automatic backups you do.