I was wondering how were you able to stop the black label from reflecting in the liquid object
I didn't do anything special about that, i'm sure the label is actually reflecting, like it would naturally do, but since the setup does not include any non-physical hacks, everything just looks normal and plausible.
how you were able to make the displacement only affect the outside of the glass?
It's combination of material ID and Corona multi-map. Polygons of the bottle has two different material IDs assigned to them - ID1 for polygons that should have knurling and ID2 for polygons that shouldn't have it. On material side, it's Corona multi-map that's defining which ID will have knurling and which will have 0 displacement. Hope that makes sense.