FBW A3nx is a special case since it started as a mod and grew. Still is a fork development of the default plane.
3rd parties normally won’t do that, they will develop their own model “from scratch”. Like FBW do with their A380X.
The core sim, like any other platform in the market, current or older, provides a bunch of default planes to enjoy the whole thing, but from that point it’s up to the community.
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