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.

no one cares about you you tube channel, this place if for learning.

the above mentioned statements talks about the creativity of an artist and to what extend they can utilize and experiment on the objects provided.   differing size and subjects are given so that an image of wisdom is created. displaying a transition from light to dark in something by darkening sections in order to create a shadow. and lastly, putting bits and pieces together to formulate a new image.