You know what? In going to go ahead and say it: Alphys made the blueprints for the DT extractor.

Let's look at the facts: Alphys discovered Determination. She named it, too.

I know the fandom likes to say that Gaster made the blueprints, but the above screenshots are proof that Determination wasn’t even known about until Alphys became the royal scientist. “But wait, Chris!” you might say. “It says ‘the blueprints,’ doesn’t that mean she used someone else’s blueprints?” And to that I say, “not neccessarily.” It’s certainly possible. But the only thing that line really specifies is that she used blueprints. It doesn’t say whose. Which means, it’s not a certain fact that they’re Gaster’s. They could have just been her own.

And assuming they were made by Gaster, then sure, he could have built the DT extractor and she used the blueprints to figure out how to use it, or she used the blueprints to build it — but if she’d done that, then why would she have named Determination? Usually people who discover something are the ones who name it. If she had, indeed, been reading Gaster’s blueprints, then she would have just discovered Determination from there. But no, she’s the one who discovered it, which means the blueprints can’t have been his. Alphys must have been the one to make the blueprints.

Also, look at the name of this thing. DT Extraction Machine. Determination Extraction Machine. Further proof that this was made by Alphys.