How do cats make friends with other cats?

Most don't, but if their raised together as kittens they will create a fondness for each other. Adult cats are territorial and generally will fight, let alone start licking each other. Adult cats won't even recognize a sibling if they've been seperated.

If you really want your cats to become friends, expect and allow fights, otherwise they will never determine their relative social ranks and without knowing rank cats can never be friends. Unfortunately fights can get messy and bloody. If one cat persists in immediately declaring "total war" on others, isolate it where it can smell them but not see them. If the cats are well "cat socialized" they will automatically limit fights to swatting and Wrestling without using claws, as neither really wants to get hurt but they can still fight hard this way. If you can't tolerate cat fights, your cats can't become friends and you will have to put up with it. There must be one and only one Topcat.