How is Activity Monitor Pulling processes specific memory information?
The Memory Use column - where is it coming?


It corresponds to memory_usage column in sys.dm_exec_sessions.

As a friendly note, I would suggest not to use Activity Monitor.

Use  Adam Machanic‘s free procedure, sp_WhoIsActive (uses DMVs). You can even log into a table for post analysis.

Kendra Little talks about Activity monitor limitations :


It groups wait types 
Leaves out wait types
Dont leave it open ... else it will boil down your server :-)