Originally Posted by jib71

Procedures for a "fully vaccinated" US citizen just means completing the necessary forms and providing proof of vaccination. For a person who cannot provide proof of vaccination and booster, the procedure includes a pre-travel test (72 hrs). In both cases there is no quarantine - no period of isolation. (Of course, you should read the fine print about what "fully vaccinated" means, what tests are acceptable for unvaccinated passengers, and what forms are required etc.)
https://jp.usembassy.gov/services/welcomebacktojapan/