You don't switch the esim on or off, you switch the line on it on or off. Subtle distinction, but more accurate since the esim is the equivalent of the sim tray. The lines are accessed on the first screen in the Settings Menu under Cellular. Touch either line and you have a toggle button option to "Turn On This Line". I don't expect 5G to be widespread in too many locations world wide for a while, and even when it is, it may not be included in the prepaid data services that are common due to possible cost factors.

I have not seen anything definitive regarding the concurrent dual sim access to 5g on new iPhones. If you have any supporting information, it would be appreciated.