The UEFI specification defines a "boot manager", a firmware policy engine that is in charge of loading the OS loader and all necessary drivers. The boot configuration is controlled by a set of global NVRAM variables, including boot variables that indicate the paths to the loaders.
OS loaders are a class of UEFI applications. As such, they are stored as files on a file system that can be accessed by the firmware. Supported file systems include FAT32, FAT16 and FAT12. Supported partition table schemes include MBR and GPT. UEFI does not rely on a boot sector.
Boot loaders can also be auto-detected by firmware, to enable booting on removable devices. Auto-detection relies on a standardized file path to the OS loader, depending on the actual architecture to boot (\EFI\BOOT\BOOT[architecture name].EFI, e.g. \EFI\BOOT\BOOTx64.EFI).
It is common for UEFI firmware to include a user interface to the boot manager, to allow the user to select and load the operating system among the possible options.