Save files on console can only be transferred between those platforms using the methods each respect console provides.
Save files on Windows, Mac, and Linux may be transferred between each other. Note that the files may not run if moved to a platform where the version of Slime Rancher installed is earlier than the last version the saves were played on.
An exception to this is saves on Microsoft accounts. Saves on Microsoft accounts may be accessed on the Xbox and the Microsoft Store version for that account via their cloud, however they cannot be transferred between any other platforms.
Please note that Slime Rancher will keep multiple versions of a save file which can be used to return to an earlier save point in the game. You will only need the file with the most recent timestamp if you want to transfer it to another viable platform.
Steam users should be aware that if cloud syncing is enabled and you are moving between multiple machines, that it will automatically override any current save files with the most recent ones it has saved.
Save file locations:
C:\Users\[username]\AppData\LocalLow\Monomi Park\Slime Rancher
Macintosh HD\Users\[username]\Library\Application Support\Monomi Park\Slime Rancher
.config/unity3d/Monomi Park/Slime Rancher