This is two three part in-depth write up on how to move files between the Windows Store version of Astroneer and the Steam version. You can also use this and omit the parts about Steam if you want to share a file from one Windows player to another. There's a link to a video guide at the bottom of this post.
To quickly locate your Windows 10 save files. Press Win + R, copy/paste the line of code below, and press enter:
cmd /c Powershell.exe "explorer ((Get-ChildItem $env:LOCALAPPDATA\Packages\SystemEraSoftworks*\SystemAppData\wgs\ -Recurse -Filter container.*).FullName | Wher