It should be simple. I subbed and downloaded a few bases (grids) of different types from the Steam Workshop. They are now in my blueprint list along with some ships that I downloaded and some that I built.

I started building a platform with a room for energy production under it on the nearby moon. Rather than build a moon base from scratch, I decided to plop down a refinery grid from the Workshop and a corresponding quarters building next to it,

After some dancing around in the dark, I finally got the refinery grid to line up with the existing platform and sit on it properly. Most of the time the hitbox was red unless it was too high or buried in the platform, literally. (Which is weird that it would give a green hitbox when half the Refinery is buried in the ground.)

Finally I got it to sit properly, but on checking the Control Panel menu in the newly placed refinery, I saw that the refinery had no idea that there was a nuclear reactor, cargo container and a enclosed step ladder beneath it. (To say nothing of the wide expanse of concrete texture grid beneath it as well.) It wasn't there, which means that its only "floating" on the platform, not part of it. What am I doing wrong?

Of course it never orients correctly on paste and you have to flip it so it lines up right. and then push it back and forth until you get a green hitbox and then click, but that's not all, is it? Why doesn't a grid attach to another grid that it is resting on?

I've never used merge blocks, don't know how they work, what parts they consist of. SO how do I make a Workshop grid attach to an existing platform so they are one piece?