[2020] How exactly do you unlock your mastery classes?

Howdy folks. I just started playing for a bit, and I've gotten a Sentinel to level 20 but I haven't yet seen an option to unlock any mastery classes. What specifically do you need to do in order to unlock them?

Apologies if this has been asked before. I did a search here but failed to find anything other than "they unlock around level 20".