Hey so if i purchase 50 extra Gb

iCloud upgrades are only for iCloud yes. If you purchase 50GB then you get those 50GB for your iCloud on a server in Apple's server farms somewhere, so you can store data there. Like device backups and other App data.

You cannot in any way alter the amount of physical storage on your iPhone. That's fixed and can never be changed.

No, it is an online synchronization system not device storage.

Yes, it would be added to your iCloud space.

It is added to your account, not your phone. If you get a new phone just sign into your account on it and you will have the same iCloud storage available to you.

Yes. iCloud Storage is tied to your Apple ID, as long as you sign in to iCloud on the new phone with the same Apple Id, your purchased storage will be available to the new iPhone.