After Arya Stark leaves the House of Black and White in Game of Thrones season 6, does she know how to access and/or make poisons?
It is likely she knows how to mix poisons.
In Ayra's training scenes during S6E3, we see her smelling some ingredients and she is able to tell aparently determine what they are by smell alone (0:55 to 1:09). We cut away from that scene but come back to it and see her mixing the ingredients into a bottle (1:44 to 1:46, 1:53 to 1:58, 2:01 to 2:04).
However, we do not fully know what these ingredients are or what they are making. We can speculate that it is poison...