Mac uses the Apple File System (APFS) and Windows uses the New Technology File System (NTFS). This first way is the inexact way, but it works. There is a clunky way to move your games and a sensible one. Here are some ways to change game download locations in Steam.
If Steam already exists and you already have games there, you can move them if you want. Many Steam users find that they run out of space on the currently used partition of existing storage or need to replace the drive due to failure or upgrade it to get more space. How to Move Game Downloads of Existing Steam Games
