I am planning a rewards flight LAX - NRT UA business class in Aug with the hopes of Japan opening up for tourism sometime in the next few months. However if it does not open by then I am planning on booking a separate flight NRT - BKK on a different carrier (JAL) the same day 3 hours after arrival in Tokyo. I understand international to international transfer is allowed but are you able to stay airside without going thru customs for lets say arrival NRT in terminal 1 and lets say departure Terminal 2 or 3 or you restricted to terminal 1 departures

Carry on bags would probably be required as checked bags would have to be picked up after customs and not possible with current restrictions