If you only want to move a few photos over from one Mac to the other we recommend AirDrop, but if it’s the whole Photos library you want to move, with all your Albums and associated data in place, we cover a few options below.
You can find your Photos library in your Home folder (which probably has your name) > Pictures > Photos Library.photoslibrary. You’ll notice that you can’t just click on that to access folders for all your photos, this is because the Photos library in macOS acts like a file when it’s actually a package, a special macOS container for holding many objects together. This prevents us from accidentally moving items out of it, renaming files that need to have fixed names, or modifying documents that shouldn’t be touched. It also makes the Photos library and similar libraries and app-associated file packages easier to move. If you do want to see the contents of this folder you can do so: press Control at the same time as you click on the folder and choose Show Package Contents.
Read more at Macworld.com