Scripted Masking, what's the approach?

Quite generally you need to understand that expressions have no magic. Anything an expression can do can also be done with keyframes or static property adjustments. Expressions merely define parametric relationships that can include basic direct links (those pickwhip expressions to rig up sliders for instance), conditional states where X happens when value Y is this and that or at the end of the day hyper-complex stuff, but with the caveat that you always need to consider that you are merely "modulating" something that exists, not creating something completely new. The lack of info is simply down to 95% of users never reaching a level of accomplishment with this stuff that would allow to create them really fancy stuff. Most just settle on using canned projects and presets from others. Similalrly, other people are more into workflow-centric stuff that is required at larger facilities or to ease the pain of repetive tasks and that is pretty much all scripting, with occassionally some expressions sprinkled in, but overall the expressions community is quite small.

Mylenium

Not sure if you're asking about coding this or if you just want to get the ned result.  Have you checked out the effects:

• Grid
• Checkerboard
• Card Dance
• Card Wipe
• Block Disolve

From what you've described I think all of these are possibilities.

And I miss Flash too - I made all sorts out of it, including a telepromter, video player for theatrical productions, an interactive "hologram" - sob

Yeah, of course you may have to get into expressions to create complex mattes for transitions. Not sure what you need to know about "resources". The online help has all the AE-specific functions listed here:

https://helpx.adobe.com/after-effects/using/expression-language-reference.html

The rest is just regular ECMAScript/ JavaScript and you imagination driving these functions just like they did in Flash. There's just more limitations. Otherwise I'm not sure what you're really struggling with, but to be honest it seems that you can't get to grips with some core concepts of AE. You seem to expect it to dynamically generate content instead of manipulating existing content and the property streams and likewise you still seem to think in Flash terms like goToAndPlay() as opposed to valueAtTime(), setBlend() vs. applying expressions to opacity and similar. Likewise, and no offense on that, you seem to expect expressions to work globally when yes, it can be as tedious and repetitive as having to edit them on their individual properties, even if it means copy & pasting them a hundred times to a hundred pre-arranged solids. That doesn't necessarily mean it has to be this way, since it's perfectly possible to create "responsive" code that adapts to duplicating layers or global controls, but most of the time you start out with a base setup before really getting into the thick of it and optimizing it.

Mylenium