You are referring to ITAR. Under ITAR, permanent residents are considered US Persons and not subject to export controls. Clearance is a totally different thing that people often confuse with ITAR, which is only granted to US citizens, and though it is possible to grant something somewhat like clearance to non us citizens, it is almost never done.
Edit: Ah I see you are not disagreeing with what I said at all. Apologies, I cannot count the number of times I have seen in reddit and here, where people confuse ITAR with clearance.
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.
By definition a US Person would not have a visa, since that qualifier only applies to US nationals, permanent residents (which do not need nor have a visa, permanent residents do not apply for admission after status is granted), refugees or asylees.