How to update latest MacBook pro EFI firmware manually?
The EFI partition is used when the firmware is updated. I assume this EFI partition must be on the primary internal disk. Apple posted that firmware updates require a GUID partition scheme at the archived website Firmware updates for Intel-based Macs require a GUID partition scheme, although the site does not explicitly say this must be the primary drive. When a SDD or HDD is formatted to use a GUID partition scheme by the Disk Utility or diskutil command, the EFI partition is automatically created.

Basically, the firmware update is copied to the FAT32 formatted EFI partition. The firmware is then instructed to update itself. Basically, the same procedure occurs for firmware updates on my HP PC running Windows. So, this is not unique to Apple Macs.

Having to update the firmware to run High Sierra makes sense. The APFS is new and older Macs would need a firmware update before being able to recognize this filesystem and read the boot files.

You used to manually update the firmware. At some point, Apple decided to include firmware updates in the macOS installs. Apple posted this in the now archived website About EFI and SMC firmware updates for Intel-based Mac computers.