Need rule clarification

[Argharg 0]

Hello, so I have bought all the extensions, all the DLCs, sleeved all the cards, painted all the minis, and now I am contemplating playing the game.. 😀

my question is simple, Assuming I play a scenario mixing the main game big box ( MGBB ) and another box let’s call it extension A, do I play will ALL the cards from all the extensions ( damages, items, and so on from all the boxes ) or only the cards from MGBB and extension A? My understandings to mix all the cards from all the boxes but I may be wrong.