(Bencow - I saw your other question about this but couldn’t log in at the time to answer)
The data from the mediabay is written into an sqlite database titled mediabay.db3 in your Cubase user settings folder - there is a shortcut to this folder in the Cubase folder on your start menu (or whatever Macs have instead), or on Win 10 it is located at User_Name\AppData\Roaming\Cubase_Version
I don’t think Cubase has the ability to merge databases. If all data is written into files then this would be picked up when you scanned the files in Cubase on a second computer - you could test this easily, but I would test it across different file types and try to identify and test any assumptions you are making.
Information in sqlite databases can be merged - there is a capable, free sqlite database browser - you could use this or another method to export relevant info from one database and write it into another - good practice would be to make a backup copy of your database before doing any editing. Since the Cubase database will have lots of entries that you do not want to edit, you might want to add an additional property to make items you wish to copy easier to find.
I am fairly certain you can work on items in Cubase LE and then bring those items into Cubase, but you will need to test this - each has it’s own mediabay.db3 file. However, the capabilities of mediabay are limited depending on the version of Cubase you are using (I think this has changed somewhat since it was introduced in Cubase 5, but I think it used to be the case that only the full version of Cubase allowed you to create your own tags)
I think that at least some of the information is written to a header within the file itself, but I’m not sure if this is true for all file types - I say this because I’m fairly sure that I have added information to files in Cubase LE or similar on my laptop, and when I’ve copied across to the desktop and scanned them into mediabay the same information has been available. I can’t remember whether this is how the broadcast wave (bwav) file standard works or if Cubase uses something like this, nor am I sure if Cubase reads id3 tag information which it seems mixed in key uses - in other words, Cubase may be writing information to files but using a different system for this that the id3 tags which mixed in key uses