So, I played the free weekend and wanted to continue playing via Xbox Game Pass. But it seems there is no way to move my saves over.
After investigating I found that the folder was in: (As per "https://support.obsidian.net/article/61-where-are-the-save-game-files-stored" says)
%USERPROFILE%\Saved Games\Grounded Windows 10
But it seems after an update in December that had changed. I found similar files in
C:\Users\User\AppData\Local\Packages\Microsoft.Maine_8wekyb3d8bbwe\SystemAppData\wgs\
But it seems that these are proprietary files made for Microsoft UWP Applications, which I'm not sure how to use.
I had also found out that the UWP version of the game has an Export button. Which I think means the developers know that people would want to transfer their saves from the UWP version to the Steam version as the UWP uses the proprietary format, but didn't acknowledge that maybe people wanted to do it the other way around.
My Suggestion/Feedback to the developers would be:
If they have to use this format. Please put a "Import" button in the game.