How to flash Magisk-x.y.z.zip using fastboot without using TWRP?
You probably meant fastboot mode.

And no, Magisk cannot be flashed using Fastboot mode. It is a zip file and not an image file that you can just flash to a partition.  If you want to use fastboot, you should consider patching your boot image using Magisk Manager and then boot that image using fastboot boot command.

fastboot boot magisk-patched.img


Or if you are sure you have the correct boot image, you can flash the patched image too:

fastboot flash boot magisk-patched.img