AIUI, and given that SSD (USB-based?) "booting" currently requires a uSDHC card AFAIK, the eprom bootloader mechanism only uses, ie. writes/modifies file(s) on the uSDHC card directly associated with the EEPROM and subsequently, during a required reboot, updates the EEPROM if/when required. Whilst "copies" of those files may be placed on both the uSDHC card and/or the SSD once the EEPROM has been updated they're not used (again) until (overwritten/replaced) a new update is required. (NB: this is my interpretation** of https://www.raspberrypi.org/documentati ... teeprom.md - I've not yet used that process only the earlier, "test", manual method: https://www.raspberrypi.org/forums/view ... 7&t=246027) To put it another way, again AIUI, it's always "both" for you, but only uSDHC card for those that don't have (parts of) the O.S. on another storage device.
** If that's wrong, no doubt someone will correct me!