The factory reset does not necessarily affect the eSIM(s): If you initiate the reset via "Settings>System>Reset Options>Erase all data (factory reset)" you will find the option "Erase eSIMs" in the subsequent dialog. If you do not select this option, the eSIMs will not be deleted and can be used again as usual after the device has been reset.
A backup + transfer to another device is - as far as i know- not possible.