I am being careful to just emphasize certain points of information that have been publicly shared in the development updates as the NDA is no joke.
XML (exactly from FSX) is present as a legacy-only feature to ease porting. It works just fine with a few minor caveats.
C++ / DLL gauges are not allowed and native modules cannot be loaded. A new gauge module system based on WebAssembly (shipped as llvm bytecode) is being built. This method is expressly to ease porting of existing C++ codebases and (eventually) provide high-performance features. If you want to try this now without the sdk to see roughly how much work you are going to need, go download emscripten and try to do a single-threaded build of your codebase with em++ with only libc/libc++ (no windows.h), gauges.h, and consider your usage of GDI+ which they have stated they are building a wrapper for. As mentioned by them, this is still heavily under development.
The "new" MSFS system is based on html/css graphics and typescript code via an embedded web engine using Coherent GT. This is not yet exposed through the sdk.
Most developers I talk to plan to take the Wasm route. I will not say with what system the existing MSFS planes are built, but you can guess.