The proper way of doing it is using what’s called a generator bypass switch, basically it’s a physical switch that runs before your fuse box, and it makes it impossible to have both the main and the generator being fed at the same time, so you can either have the main on or you could have the generator on. This prevents the electricity from your generator back feeding into the line and killing a line worker trying to restore power.

Sadly, like the other comments have said people tend to use these male to male cables in order to not have to pay the $2,000 to install the switch and instead choose to just turn the main breaker off and plug that cable in. But since it’s possible to have both the main and the generator on it’s not legal because if you forgot to throw the Main or if you did it incorrectly you could be putting workers at risk

Even disregarding the safety risk of using such a cable, not having a dedicated switch installed also means that you’re plugging your generator into usually an outside socket of the house, and those power lines aren’t usually meant to have a high load so you risk creating a fire from over straining the line as well