what does this mean syncing photos via finder is not supported when icloud photos is turned on? photos synced from finder will be removed

It means you can’t sync to the phone using two different methods. You either have to use the old-school method or iCloud Photos. Since these files have been synced from the Finder, it is assuming that they are still on that Mac. Double-check that they really are on your Mac first. If so, you can turn on iCloud Photos on your Mac and they will upload. Then you can turn on iCloud Photos on your Phone, deleting the Finder-synced photos, and all your photos will then be available on all devices.

So if I were to use iCloud for photo storage, I lose the ability to have a local backup of my photos? That can't be right. What if the internet is down and I want to view my photos? Likewise, how would I view and edit photos taken from my iPhone Pro on my laptop if they're not synced to my MacBook?

Scottsquestions wrote:

So if I were to use iCloud for photo storage, I lose the ability to have a local backup of my photos? That can't be right.

It isn’t. You can still have local backups.

What if the internet is down and I want to view my photos?

View your photos. They are still on your phone. They may be in “optimized” versions though.

Likewise, how would I view and edit photos taken from my iPhone Pro on my laptop if they're not synced to my MacBook?

If you don’t have iCloud Photos enabled on your Mac, then you would have to manually copy the photos to your Mac using AirDrop or similar. But the idea is that you would have iCloud Photos enabled on your Mac. If you make sure that “Optimize Mac Storage” is disabled in Photos > Preferences > iCloud, then that is your local backup.