Getting MSFS through Steam, after install, does one always have to start the sim though the Steam app or is there a standalone exe file to start MSFS with after you install it? Also, does the Steam app need to stay open during the sim session? I keep hearing about how purchasing it through Steam is a much better experience for updates down the line so want to go that route but have never used Steam before. Thank you 

Steam has to be open, yes but that is no different from all the Microsoft store / services running in the background by default. There is a good ole' standalone .exe, if you use that, Steam opens up automatically if you haven't done so. I think the purchase / update experience is generally on par between the stores, as the bulk of the update process is done within the sim itself, so it's identical. Only major pro of the Steam version for me is the fact that the .exe is not encrypted, allowing stuff like ReShade to work flawlessly. 

Use Steam to make a simple desktop shortcut, that's all you need.  Runs MSFS straightaway.