How to copy your Photos library from one Mac to another

You can avoid most of the Internet bandwidth overhead by copying from drive to drive.

The Photos library used 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.