This is an excellent reply and summary of all the various APIs, thank you very much!
Just to add some “background info”: WASM stands for “Web Assembly” and is essentially a “web standard” (coming from the “browser world”) which defines a compatible “byte format” which can be interpreted by various browser (and obviously also “non-browsers” like Flight Simulator 2020) such as Firefox and Chrome etc.
(As such, it is very much in analogy to “Java Byte Code” which can be interpreted and run by any Java Virtual Machine (JVM)).
WASM is not a “Microsoft invention” and for sure it is not Flight Simulator 2020 specific: it is really a “global standard” which allows to exchange “byte code” to be run on various platforms (again, mostly browsers, but also other applications like Flight Simulator 2020 and others).
There exist many “WASM generators” (compilers), specifically also for C/C++, so you can essentially write “web apps” in C/C++ and have them run in a browser (or as an addon in Flight Simulator 2020). But as already mentioned those WASM modules are in a tight (hopefully ;)) “sandbox” (which might be specific to the context which executes the WASM code), so they have only access to a very limited API (which most likely excludes most operating system APIs, for instance).