I thought the US had extremely strong constitutional rules about not using the military on the US mainland?

Whatever the rules are, I am guessing they are relaxed around civil operations. Pararescuemen (USAF PJs) were deployed for search& rescue during hurricane relief operations.

More if you want to dig into it: https://www.defense.gov/Newsroom/News/Article/Article/164568...

I don't see any mention of that

https://www.archives.gov/founding-docs/constitution-transcri...

I think it's called the Posse Comitatus act or something like that.

It looks like that specifically forbids them from enforcing the law, not public works projects. And there seem to be several exemptions.

https://en.wikipedia.org/wiki/Posse_Comitatus_Act

We have Army National Guard that are often deployed during weather emergencies.

National guard are state resources whenever the feds aren't using them.