Looking at others’ photographs and trying to do what you saw is okay. Why isn’t this plagiarism?

Because you are not claiming it as your own work. It's more of practice. In an art class, all the students try and recreate what the teacher made. It's the same thing.

the mass and the sacrifice are similar because jesus christ is the victim and priest in both. the mass is different from the sacrifice in that jesus died for the remission of sins and cannot be  sacrificed again during the mass.