The selection doesn't appear to be random
I never said it's just random. It's random
is random amongst all the ones that matches all the constraints which is also why I linked to a post that even explains where the constraints are and how to change them.
since the update I have not seen the old non-motorized steps (or the older motorized steps for that matter) on a 737 once in multiple flights
If you mean the PREVIOUS 2.9.2 update then yes, it was random ON A 737, because so many people complained about getting the "wrong" (wrong = unmanned) stairs on front that we thought most users would be happy to have the new much better looking model that IS used in real life on a 737 so, we set NON-OVERALAPPING constraints, so a 737 would only get the new stair.
Apparently, somebody liked the old one as well, so in the CURRENT 2.9.3 update, we changed the constraints (see release notes), so now you will have again a random selection between the old and the new stairs on a 737, because now their respective height constraints OVERLAP.
Is there a way that we can manually select these to appear for the 737 instead of the new steps, either in menu or in the file system?
Again, I linked an old post which explains how to do that, yes it's for P3D, but the constraining rules are identical so, you can apply the same principle to MSFS, the only difference is the various SIM.CFG files are in the \Addon manager\MSFS\fsdreamteam-gsx-world-of-jetways packages Simobject folders and, in MSFS, you must clear the model cache each time you make a change, then restart the Couatl engine.
And of course, the next GSX update will overwrite you changes.