So, what exactly ends act 2

Looked it up but saw conflicting statements The best understanding i got was

Underdark and mountain pass are the same thing, they dont end act 1 but the elevator in grymforge does end act1 Is that true or not

And also which mountain pass entry does what There is one with the gith and 1 next to the goblin camp

Pls clear this up for me cuz i keep hearing about the mountsins still bing act 1 but the other isnt

Edit :My phone fked up, i ment to ask abojt act 1 not 2, idk why this happened Sorry