If you do understand First Amendment then you should also understand that foreign propaganda is protected speech, and is not treated as yelling fire in a crowded theater:
https://firstamendment.mtsu.edu/article/lamont-v-postmaster-...
https://firstamendment.mtsu.edu/article/lamont-v-postmaster-...
But spreading foreign propaganda is indeed illegal despite that precedent if one does it as an agent of a foreign government within the FARA legal definition (which is reasonably implied by being on their payroll) and does not register with the US government as a foreign agent, aside from certain exceptions.
That’s the scenario which started this subthread: