They put out a whole playbook called Project 2025, developed by a bunch of insane people at the Heritage Foundation. Trump lied and said he didn't know anything about it. Now they're doing it.
Not only did they put out a playbook, they actively recruited and screened people to take government positions. I read a post on Reddit from someone who went through the training. The whole idea was to find ideological supporters. Trump's problem in 2016 was that he wasn't prepared and relied a lot on existing government supporters and established GOP figures, who weren't completely loyal. This time, they were prepared. That's the reason why he's been able to move so quickly in the first month.
Actually, he said during the Kamala debate that it had "some good ideas and some bad ideas" to my recollection.