Iphone storage vs Icloud Storage

iCloud stores your photos in their original state however depending on what setting you use on your device, e.g. if you use the Optimize Phone Storage feature, a smaller version of the photos will be stored on-device to conserve space - Manage your photo and video storage - Apple Support.

Johnny B 81 wrote:

That's ok, but how much space it will use? All my Iphone space (and I will have a full device) or it will keep some free space? I asked this question because in this moment I use a NAS to backup all my photos (350 GB of photos) but I was considering the possibilities to migrate everything on Icloud Photos. My fear is to fill all my Iphone memory, creating a problem to myself instead to solve it

The amount of space used on your device is a variable. If optimized, it is typically about 10% of the iCloud space used but is not assured.

This is also limited to space on the device It is possible your library will grow to a point where you don't have enough local device space for even an optimized library. You can't jam 10 pounds of sausage in to a 5 pound skin.

There is no cloud only option using iCloud Photos. To some degree, your photos will always be present on both device and iCloud. It is a syncing service and not off device storage.

What do you mean what happens to the Photos app on your device? That's where you'll find your photo/video library.