How do you know if you can trust someone that way without dating them in the first place? Only date people you've been friends with for several years?

Yes, actually.

I'm curious about this - is there some formal unveiling of passwords as a relationship milestone? Or does it just come up naturally, like, "I need to log into your gmail account, what's your password?"

Well, whenever one of us gets a new phone, we put the other's fingerprints in, just as something we started doing out of convenience. After that, yeah, pretty much. We don't share all passwords, but we would if we needed to.