Welcome aboard! Arno has been very diligent about trying to keep MCX relevant. Currently MCX will not recognize "Behaviors," which are almost exclusive to scenery for all but the most complex objects, like a seasonally conditional visibility, or a wind driven sock. Also skinned mesh animations are not handled, so the workflow for animated people must be through Blender or 3ds Max. Ultimately most scenery files within the sim are compiled to .bgl format, however there is no official decompiler, so all development work is done in XML, gltf, .png and other exchangeable formats.
MSFS does use .DDS, but again, it is employed in such a way that it becomes a proprietary format. Like the .bgl conversion, .DDS is applied to .png format textures at final compile, what Asobo calls the "Build" process. Along with format conversion, a compression algorithm is applied to "improve brightness," despite the fact converted COMP textures are visibly reduced in contrast, see below. As to the old texture slots, only albedo and normal remain, attributes called anisotropic occlusion, "roughness" and "metalness" (mnemonic "ARM") are converted to grayscale values and applied to the red, green and blue channels of what Asobo calls a "COMP" texture, to be Asobo's version of PBR materials. If we're using MCX, we place the COMP texture in the metallic slot.
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This image was made by comparing default DevMode sample project textures from the Simple Aircraft project, with textures that had been compiled to .DDS by FSPacagetool.exe and then manually reverted back to .PNG using the Nvidia standalone converter.
ADE remains relevant in MSFS2020 and was indispensable for my transition, I'm still unsteady creating taxiways using the Scenery Editor and prefer using ADE to do so, probably out of nostalgia. That said, the Scenery Editor is extremely powerful, slick, WYSIWYG, it's something you need to experience, after all your years under the P3D/FSX hood, so to speak, sir. Yes, jetways are in an XML written document called "airport.bgl," that in structure is identical to what you're calling "AFCAD."
The folder hierarchies are fairly similar. the "Community" folder is like a supercharged "Addon Scenery" folder and the hardest thing to get used to, I think, is that we don't work back and forth out of our "project" and "published" folders. Basically, you work on something and then you fire it off to the Build, test it in sim and if it doesn't look right, go back to your source files. This isn't to say there isn't real time editing and this is what is kind of crazy about the sim, it
watches us. So if you're working on a model in I don't know, I was always using Sketchup and then you run it through MCX to get it all squared away with GUID and PBR and all that, you compile it into your working project folder while the sim is running and the sim will update that model in real time.
This is kind of a side effect of the Virtual File System that Asobo uses to protect from viruses and malicious code. Since it is building all files virtually, when it detects a change in the source files, it updates the changes. In my experience it's not 100% reliable, it seemed to cut out after 3 to 5 replacements, other people have suggested that textured polygons, as opposed to colored polygons complicate or prevent this refreshing process. Still, it's pretty cool when you know about it and are anticipating it.
I have 3 published sceneries, the one in my sig has a user activated hangar door, also a working wind sock, some of my others have custom AI boats and planes, besides those, please take the time to review the SDK. Nearly everything about MSFS is hacked from FSX and the MSFS SDK really helped me compare what I knew, to what they'd changed. The SampleScenery project is key to learning how to place objects, imo.
