How to disable the physical sim and only use the eSIM on iPhone XS/iPhone XS Max?
I found the option buried under Mobile Data Settings. There is an option which states "Turn On this line". Toggle this ON/OFF to turn off the network.
This works for both Physical SIM and eSim.

At any given time, You could -


Have Physical Sim ON and eSim OFF.
Have eSim ON and Physical Sim OFF.
Have both the Sim turned OFF.