Pass out and faint can mean similar and totally different things. Pass out and faint can both mean to lose all consciousness. They also have their own meanings.
The meaning of " to pass out" in this case is the verb of faint. To pass out can also mean handing out objects or information. If someone passed you something, they handed you something. The phrase " to pass out" means "to let go of"
Faint can also mean something that is barely noticeable or unclear.
" The smell of the rose was faint". " The noise was faint." The word faint means "weakness".
With those definitions let's picture why they are used. If some lost all consciousness and fell or their eyes rolled into the back of their heads. They are letting go of even if it's against their wishes. Their body is letting go of due to whatever reason. They passed out.
If someone lost all consciousness it was because their body was weak, they were faint. That's why we say " fainted".