If you buy more iCloud storage does it mean you can have more storage for photos and videos for your camera roll and more space for some apps and music?
No.
iCloud memory has no affect on the storage capacity of your iPhone.
Depends how you use it. If you use Apple Music and iCloud Photo Library then yes, as the music and photos will be store int he cloud rather than on you device. You'd then stream your music, and only small preview images would be save to the device, rather than the HiRes versions thus freeing up storage.
Or if you just used the iCloud for storage and backups then, no it will not free up space on your device.
If you buy more iCloud storage does it mean you can have more storage for photos and videos?
