The EM7345 is only supported through the OEM vendors distributing it, like Lenovo.
Note that many of the commands you see are generic chipset (or baseband firmware) commands, common to modems using the same or similar chipsets. I believe the “AT+X…” prefix is typical for Intel/Infineon chipsets. You should be able to use docs for these chipsets or other modems using the same chipset. The EM7345 use an Intel XMM7160, I believe,