In the past (in Windows at least) one could open a .manager file directly by double-clicking the file or following the Open With dialog but it doesn’t seem possible anymore. This was useful for opening old backups rather than importing the business file using the Add Business > Import Business dialog in Manager and then having to remember to remove it again.
The only other option is to follow the Change Data Folder menu item and temporarily use the folder with the .manager file one wants to open and then change it back to one’s main working folder.
I also tried to run Manager by extracting the appx file and running the manager.exe but that still didn’t allow the direct opening of the .manager files and merely opened the default Data Folder.