where do you find the password for the remnant bunker? do you get it from finishing a main quest?

im worried if its just found in some random spot or obscure quest, ill overlook it...

It's related to one of the companions

It's related to the worst companion.

oh gay. i dont wanna give up ed-e and boone forever

You an regain any Companion. When you tell them to leave, they go back to where you found them. You just go find them again and talk to them to have them rejoin you.