The Boron LTE (BRN402, LTE Cat M1, with u-blox SARA-R410M-02B) is only supported with the built-in Particle MFF2 SIM in the United States, Canada, and Mexico.
It is possible to use the Boron LTE with a 3rd-party SIM in some cases. Telstra is one that currently works, however you need to manually configure the MNO (“Mobile Network Operator”) profile using AT+UMNOPROF. Telstra is profile 4.
In other cases, such as Verizon in the United States, you cannot use a Verizon SIM or a provider that can use Verizon (like Hologram), because Verizon LTE Cat M1 requires certification of the device and registration of all certified device IMEIs, so after a few days, the device will be blocked from the network if not certified.
No current Particle cellular devices have programmable eSIMs. The ones with built-in SIMs (Boron, B Series, Tracker SoM) are MFF2 SMD SIMs, which are basically the same as 4FF nano SIM cards, except in a smaller solderable SMD form-factor. They cannot be reprogrammed to other carriers.