Though Steam has cloud sinking, there are times when you may need to manually move a game save file to another device, or maybe to share it with another player entirely.
Luckily in Rimworld this is very easy to go.
With the game open, go to options
Then Open Save Data Folder:
Which will bring you to your file explorer window, and straight to the right location.
Go into Saves, and choose the game you want to transfer (it’s best to make a copy of this file and save it somewhere easy to find like your desktop, so if anything happens you still have your original file.)
On your new file, right click, select ‘Send to’ and ‘Compressed (zipped) folder’
which will get you a neat new folder.
You can then send this zipped folder by data transfer method of your choice; Discord, carrier pigeon etc.