What is the difference between twrp-[version]-[phone].img and twrp-installer-[version]-[phone].zip?
The answer becomes clear once you read the install instructions:

You will need both files to install, as your device (OnePlus 6 aka enchilada) is an A/B device and thus has no recovery partition (recovery is part of the boot image there) – which means you cannot simply flash the recovery image using fastboot. So you download (or push) the ZIP to your device, then temporarily boot TWRP (fastboot boot *.img) and from there flash the ZIP.