First off, I am not familiar with the FS 2020 SDK when it comes to creating simulation objects (like aircraft): I only know the API “from a client perspective”, notably the SimConnect API.
That said, I have been told that you can create so-called “local variables” (“L:vars”) and use those, e.g. for custom animations of certain aircraft parts. In fact, there are 3rd party aircraft which just do that. Probably the most prominent example is the A320 by FlyByWire.
Here’s the catch: those “local variables” are not (directly) exposed via the SimConnect API! That is the reason why aircraft like the FBW A320 stopped working reacting to simulation variables which control e.g. the engine thrust etc. (because they are using their own “local variables” now to simulate the engine, or so I understand).
That is, there is no “official API” to access those local variables (identified e.g. by their name or so). Or not that I know of, at least.
However there are approaches (already discussed here in the forum) how you can access those local variables (for both read and write) via the SimConnect API, essentially via “custom events” (if I remember correclty). The drawback: you need to register your custom event IDs with SimConnect, and those registered custom event IDs might clash with IDs registered by other 3rd party apps (extensions) that run at the same time. And needless to say that those solutions are then “aircraft specific”, as the local variables are specific to the simulated object (of course - because they are “local” ;)).
And the second (potential) drawback: you also need to install an extension (a WASM module?) in FS 2020 itself, which then “reacts” to your “custom events” (as sent by the 3rd party app, via SimConnect).
Also refer to this thread there:
(and all the 100+ follow-up posts ;))