Steam Savegame location: %LOCALAPPDATA%\ASC\Saved\SaveGames\FullGame\Save.sav

Microsoft/Xbox PC Game Pass Savegame location: %LOCALAPPDATA%\Packages\DONTNODEntertainment.Ascension-P9_1t8sv8k1cftdw\SystemAppData\wgs\

The save from Xbox is getting synced to PC Game Pass.

First start the game on the platform you want to import it and exit the game. Make sure the Main Menu shows the "Continue" option and can be chosen. Then the basic save file with its location was written correctly.

In the PC Game Pass Save Folder get into the subfolder with a long number that provides 3 more subfolders. Each subfolder is for one file. One is for the Save (Save.sav), one is for the Machine Settings and one for the User Settings. You only need to find the Save.sav file.
To do that open each subfolder and then open the file with a Text Editor like Notepad with a long number that is at least 10KB of size.
For reference, my basic save file that only included the start of the game was 10KB and the save file in which I was in Chapter 4 was 70KB.
In the Text Editor the file should show the first line with the word "ASCGameSaveData" included as such:
@DNESAV@  ASCGameSaveData ð  5.1.1-0+Unknown  MapName

Copy this file to the save location of your prefered platform and rename it to either Save.sav on Steam or the cryptic numbers for PC Game Pass (mine was 2735D034F0784490A8C0ECD13105CA5C)

After starting the game you should be able to continue that save.