The modding community for FSX/XP11 is similar to that link - just tons of content created by anyone willing to put the time into it.
There definitely is a quality scale. Free planes that can be downloaded are usually low quality in terms of system operations (things like engine behavior, fuel mixtures, etc.) however there have been a few golden eggs such as the ZIBO 737 for X-Plane 11.
Then there are (usually) payware aircraft that are very realistic in terms of systems. These are the planes that you purchase, install, and then spend 5 hours watching YouTube tutorials and reading the giant manual to turn on the fucking light switch so you can see inside the cockpit. Then the light illuminates the beautiful interior modeled to be as realistic as possible and you realize there are 25 other critical switches to figure out so you watch another 5 hours of YouTube, etc. The first "study level" aircraft I bought I remember being mentally exhausted by the time I was ready to push back - didn't even taxi the first day but it was highly rewarding once I got it going.
Outside of airplanes - the modding community is also amazing. Anything you can think of - graphic enhancements, airline simulators, mission modes - you name it. On FSX you could buy a 30 dollar add on that would simulate passenger plane activities - you sit there while they load up the plane cargo, board the passengers, and then you work your way to take off and head to the destination. The entire time you are being graded - for example if you fly erratically the passengers will complain. For me it was an OK add on - the passengers would complain about shit like the snacks weren't served early enough or the turbulence was too rough. Instead of working on improving my "airline" I just flew the plane into the mountain so they had nothing else to complain about.