The current procedure is the same as pre-Covid. When arriving in Bangkok, there is a transfer desk, where you show your onward boarding pass. You then move into the transfer Immigration area, where you are stamped into Thailand and then move on into the Domestic terminal, where you then head to the gate of your next flight.
Please note that previously security checks were done straight after immigration, which led to long queues, often back into the immigration room. That now has changed and the security checks are done prior to the gates, but this can also cause delays, so make sure you head to your gate with plenty of time.