What is the question? I have been to Japan this month. You still need triple vax or a pcr test to enter Japan until May 8. There is no test or vax required to transit.
Self transfer is where you bought separate tickets on different airlines, requiring you to complete all entry requirements in NRT. Or, those itineraries sometimes offered requiring you to go between Narita and Haneda airports in Tokyo by ground transport. Then you need triple vaccination or a PCR test. All of these rules will end May 8.
You said you are changing airlines. But is it a code share flight where your you on a single itinerary with one PNR ? Then you do not need to do anything. Japan has never required testing or vaccination for transit/connecting flights for the duration of Covid.
Go book a flight on Lufthansa from BKK to US. While you are in the transit area business lounge in Frankfurt attack somebody. Tell me what happens next?
I read this debated before. I never left the final day but I had an experience one time on 30 day visa exempt status. My Northwest flight suddenly canceled so, went back to a hotel . They did not stamp people in again they "cancelled" the exit stamp! This took hours for a near full B-747. Assume then the stay to last day types would then have been on overstay.
The airside departure terminal is simply a section of the airport restricted to employees and passengers who have completed immigration checks. The idea it is some kind of no mans land is incorrect. Some people think foreign embassies are sovereign territory also. Nonsense, from the bar-stool.