Large companies (think FAANG) can and do have non cleared, non citizen, non us persons (permanent residents, refugees and asylees) working on such projects with the proper separation of scopes, data access restrictions and so on.
One of my old managers has a story about how he was tasked with documenting an unspecified something for the US government. Because he was not a US citizen (though he was a citizen of a very close US ally), he was then no longer permitted to read the document that he wrote.
Probably just lawyers being lawyers, but still pretty funny.
