Do shared photo libraries end up on your device also?

A shared photo library (your own or one you join) apparently follows the same location rule as your private library - residing entirely on your device if you use the Download Originals option, partially if you use the Optimize iPhone Storage option.

Are you already using the "Optimize iPhone/iPad/Mac Storage" option for iCloud? If not, enable it in the Setitngs > AppleID > iCloud > Photos. Then iCloud will manage your storage on your devices. The iCloud will keep smaller, optimised versions for browsing on your device, but also the most recently used images and videos in the full resolution. You should not have to worry about storage, when you are using "Optimize". iCloud will free storage automatically, when you need it. You just have to take not to keep more items in iCloud Photos than will fit optimized onto your iPhone. Even the optimized items need local storage. How much storage the optimized versions will need, depends on the image format. As a rule of thumb: expect the optimized versions to need 10% to 20% of the size of the of the originals in iCloud. If your iPhone storage is always so full, that you can no longer work with it, your iCloud Photos Library may already be too large. For example, if you have an iPhone with 128GB storage and can afford to use 60GB for images and videos, keep no more than 300GB of photos and videos in iCloud. The local storage of your smallest device is the bottle neck to consider, how many photos and videos you can keep in iCloud Photos.