Use visibility User by uid

Very frustrated beginning user here. I'm not trying to write any PHP. I am just trying to use the provided functionality, I think. I want to control visibility by user id, which seems to be possible on the basis of a user's uid. But for the life of me, I cannot see where to view or set a user's uid. Also, I keep losing the page where I was trying to set the visibility filter and have trouble finding it again. It would be nice if the search function could show the way. Perhaps, a functionality I should request?

Any help will be greatly appreciated. And, yes, I did search for answers. I searched "visibility",  "user::uid", "uid", and others.

Thanks in advance.