Reading the UEFI booting from Wikipedia:
Unlike BIOS, UEFI does not rely on a boot sector, defining instead a boot manager as part of the UEFI specification. When a computer is powered on, the boot manager checks the boot configuration and, based on its settings, loads and executes the specified operating system loader or operating system kernel. The boot configuration is a set of global-scope variables stored in NVRAM, including the boot variables that indicate the paths to operating system loaders or kernels, which as a component class of UEFI applications are stored as files on the firmware-accessible EFI System partition (ESP).
UEFI relies on a "boot manager" which check the configuration (set of global-scope variables stored in NVRAM), this configuration includes boot variables that indicate paths to OS loaders/kernels, which are components of "UEFI applications" (which are stored as files on an ESP - aka GPT Partition(1) using FAT(12|16|32) File Format)
Looking at the Boot Manager section of the UEFI specification:
GPT containing ESP:
Figures found on page 117,118 of the specification