To start with, here's how Oxford Languages (via Google search) defines it:
- creative art, especially visual art whose products are to be appreciated primarily or solely for their imaginative, aesthetic, or intellectual content." the convergence of popular culture and fine art"
- an activity requiring great skill or accomplishment. "he'll have to learn the fine art of persuasion"
That can still be subjective, and I'm sure we've all seen photography exhibits in which work was displayed that was well within our own skill level and made us question whether we too are fine artists.
In my case, I'd guess not, because I have never even pursued it. That's easy.
Others might define it more simply as "if a museum would buy it and display it, then it is fine art".
I like the Oxford definitions above, esp. re. needing great skill. So for example if I go to the Art Institute of Chicago, I can see some paintings by Rembrandt, which I consider fine art. If I go to another part of the museum, there are paintings where the "artist" just flung paint at the canvas and somehow convinced the curator it was art. I don't consider that fine art.
There are many levels between, and parallels between paintings and photographs. Some of the photos I've seen are not very good, in my subjective opinion, but mine isn't the opinion that counts. ;-)