i want buy more storage

You meant iCloud storage ? Yes you can just choice the storage plan you want or get  ONE where you can pay les money for 50 gb of iCloud plus Apple Music ,  tv+  Arcade games or only the 50gb of storage bug if u wanted say iPhone storage that isn’t possible because that’s is iPhone physical or internal storage

The page linked to at the top of this page tells you how to upgrade your iCloud storage: the Apple One option is worth it only if you want the other facilities which are included.

iCloud storage is on the server and is accessible to your phone and any other device signed in, for syncing data such as documents, email, calendars and contacts. All data therefore exists both on the phone and online, and it does not help if your phone is short of storage.

As Ozzie1910 points out, you cannot increase the actual storage on the phone. If you are running short here your only options are to remove some data or to buy a new phone with more storage.