After you aquire ten skill points (through exploration or beating bosses), your inventory screen will suddenly have a new slot for an archetype engram.
Yout get archetype engrams by getting Wallace to craft them using some conspicuous item you have picked up.
For example, you can buy a Medic Pin from Dr Nora; you can take the Medic Pin to Wallace and convert it into a Caduceus Idol; and then you can equip the Caduceus Idol in the new slot in you inventory to dual-class with your starting class and the "medic" class.
More info about engrams below. Beware spoilers for non-starter classes.
https://remnant2.wiki.fextralife.com/Engrams