Where are the photos stored

Some of the answer about where the pictures are depends on how you have it set up. If you use iCloud to sync your devices, then the picture files get transferred to iCloud, and the Mac will keep only smaller copies (if you choose Optimize Mac Storage in Photos' iCloud preferences.)

If you don't use iCloud, then the photos are copied into the Photos Library, which is usually in the User's Pictures folder. But this Library can be located on an external drive, if you want to save space. (For safety, it probably shouldn't be kept on the same drive as the pictures came from. Having multiple independent copies is important if don't keep photos in iCloud.) This kind of library is called a Managed Library because the Photos app manages them. In Photos Preferences you must have "Copy Items to the Photos Library" checked--it is by default.

Most people use a Managed Library, but you can have a Reference Library, with "Copy Items" un-checked. Then Photos leaves the pictures where they came from, and you have to make sure Photos doesn't lose track of them. This bypasses the greatest power of Photos, which is its ability to manage pictures, edits, thumbnails, and keep them all straight. Very few people use a Referenced Library,, and some come to regret it.

Also, you don't have to have just one Photos Library. I have a bunch covering different things that will never mix. You can swap between libraries and even merge libraries if you wish.

I don't know. But I see that you're using iCloud Photos and have Optimize Storage on. So there must be a lot of back and forth between your Mac and iCloud, and I would expect the amount of space used on your Mac to be changing constantly. It may take a while for the storage to stabilize, if ever.

And you have to wonder, in such a dynamic process, how storage is calculated. Does "available" storage mean bytes available for use right now or potentially available after stuff is moved around a bit? The space numbers are at best estimates, and I expect storage estimates made by different processes to differ by gigabytes, and for that to change in time.

I remember, the recommendation to leave at least 15% of the space on a hard drive free-- that's to accommodate all of this dynamic activity and these uncertain estimates.