Hi all - I typically buy offline (old habits) but if it's a cheap and quick scenery or something I'm happy to just buy in the marketplace. I don't really pay attention to what account I'm logged into (just some random 'player' username) although all the charges go through steam (which I know my log-in of course since I bought via MSFS 2020 via steam). Do I need to think about what my log-in/username for the marketplace is or will it just continue to link bank to my steam account in future so I don't need to worry? 

I don't use the steam version but I am pretty sure the marketplace uses the Xbox account you are logged in with in MSFS.
You probably can reset your password if you have forgotten it but if you don't know the name of your Xbox account nor the e-mailaddress it is connected to you might lose it all.

Thanks, all I can see/remember is my name in the top right of MSFS 'Player6XXXXXXX'....but if I had to authenticate with an email or password at some stage I guess I can try and guess if I ever need to log back in again. I don't really want to risk signing out in case I can't get back in lol. 

Steam is only a source for MSFS and funds to purchase addons. To use MSFS you have to log in with your Microsoft account (@hotmail/@live@/outlook etc). That account is what holds your Marketplace purchases. On windows 10 and 11 its quite possibly the same Microsoft Account you're signed into Windows with. You can see this by right clicking the Windows/Start button - > Settings ->Accounts (Windows 10 and 11) .