Long story short…
Consoles have always had a much more limited access to User Generated Content for a number of reasons.
The longer story…
With regards MSFS2020 Microsoft/Asobo have provided an in-game marketplace through which in house as well as 3rd party content can be purchased and used. At present this is the only avenue available to the Xbox as it is not possible for Xbox users to purchase from external stores and import those into the Xbox as a PC user might. This is simply the nature of using a closed ecosystem.
Over time the aircraft and sceneries etc available to the Xbox market will increase. Microsoft can be quite slow even updating products in the marketplace for PC so a little patience will be required.
I am absolutely sure that more products will come, after all access to the potential market that Xbox represents is attractive to developers. Give it time.
The bad news?
Naturally there are some things that may require other external softwares in order to function correctly. These can be made to work on a PC but because of the nature of the Xbox these cannot be made to work for that platform.
Also unless Microsoft can allow the Xbox to access to free upload sites like Flightsim.to and download/install external content it will not be possible to enjoy the community created freeware which is hosted on such sites. But this should come as no surprise as it’s pretty much always been like this for console’s.
(Disclosure - I game on both PC and Console. Adding mods to console versions of games I have owned on both platforms has always been a lot more limited than on PC…or not possible at all)