At risk of seeming blind or oblivious, I have a question. How do you use Modular Constructors?
I at least skimmed every page on this thread, read all the patch notes, looked on the main page under "guides", Tried to install it in my Heavy Industry, checked if there was a special project for it, Noted that the "help" button in the bottom left of the research screen seems to be grayed out, checked the Galatia Academy, and left all of them in the Research Storage box on a colony. What am I not doing, or where do I go to get answers?
First you need to hit up pre-collapse facilities like you already seem to be doing to get (most likely) dormant modular constructor. The dormant ones aren't immediately useful.
Second you either need to get lucky or build the digital consortium industry. Lucky means that you've gotten a modular constructor that isn't dormant and instead attuned to an industry type. Going with your heavy industry example, you will need a dormant constructor - heavy industry.
If you don't get lucky you'll have only dormant modular constructors, these are useless on their own and can't be slotted anywhere but can be, with the digital consortium, a relevant colony item and a month of time be turned into the mining, fuel production, heavy industry etc. variants of the modular constructor. For this option just click the digital consortium structure and click the blue option.
As for various modular constructors, they each slot into their respective industry types, they don't do anything by themselves but the higher tier upgrades for various industries that you research will often demand the appropriate modular constructor to be installed in the industry before you are allowed to upgrade it. So for your heavy industry the item won't do anything, but upgrade it to an orbital works then look at the available upgrade for that and you'll see what I mean.
Edit: Note that the digital consortium needs to be researched before it can be built.
(edit: or is it?)