what is the download mode
Download mode is for flashing radio firmware/ROM upgrade through official means.

Some devices uses U-Boot over the generic boot, it is a boot-loader code found in read-only ROM chip on the board, which uses a certain memory address offset, in which the kernel from the /boot partition gets loaded into that specific certain memory address offset and jumps into that address and the kernel starts running.

For details of the generic LK boot which is employed by most, if not all, Qualcomm based devices, see this 

As for Download mode, for example ODIN is commonly used on Samsung devices to flash ROMs, those devices needs to be in download mode prior to flashing. However, ODIN is not the official way to do it, rather Kies, is the official route in upgrading the firmware, this is for Samsung devices for example.