How does proving vaccines work?
Japan has an electronic health registration system for all visitors entering the country. Its called Visit Japan Web (link on top of the thread) Theres a lot of videos how to upload your passport & vax records but the summary is
If you have a all 3 vax= first 2 and booster you use the site to upload a photo or pdf of your CDC card or immunization record
if you are unvaccinated or your vaccinations are incomplete (lacking the booster or only in dose 1) then you need to take a PCR test at least 72 hours before flight and upload the results showing negative result from the lab. You basically upload a photor or PDF of this to the site. Failure to do so means you wont be allowed in the country.