The datasheet above shows different pin configurations for SD and SPI modes on either SD or microSD cards. Does the built-in microSD slot on the Raspberry Pi 3 B use SPI or SD mode for activities like booting, writing, reading and more?
I don't know.
The early model Pi schematics show SD connections to GPIO 48-53 marked as SD.CLK, SD.CMD, SD.DAT1-SD.DAT4 which suggests SD.
However the same GPIO may well be used in a SPI mode during boot.