take your passport. You can have either a physical or esim. They'll do everything for you. What I did was get the SIM for 50 baht and then downloaded the AIS app . Then I looked online ( not on the app ) and topped up the SIM with the 50 gig contract. Again they'll do it . I did all this in the AIS shop in Bangkok at Sian paragon.
I flew Thai airways from the UK and they will ask for onward travel either at the counter or check-in. If you have an onward ticket in less than 60 days to somewhere else you should be ok..