They tell Hard reset removes everything from our Android Phone then how is it keeping the custom-rom zip file after hard reset so that we can install the custom rom on to the device?
I didn't try but got the doubt when I am reading online on how to install Lineage-OS
Hard reset removes everything that you'd placed in internal memory, including the .zip file. The instructions you linked assumed having an sdcard available. If you don't, you'll need another method, for example, using adb to push the zip file to the device after you've done the reset and booted into the custom recovery.