It does a pretty good job of optimizing, too. You really don't have to worry about it.
Generally if you're an average user that takes a decent quantity of photos, the last few weeks of photos will still have the originals on your phone and it won't have to get anything from the cloud to view or send them. For me, I only get the iCloud download icon when I'm working with a photo I took more than a couple of months ago.
I have a 256GB 11 Pro Max and Photos is only using about 2.5GB of space on the phone for a 250GB library in iCloud.