archetypes are more of Pf2e’s way to do ‘multiclassing’, where you choose feats from an archetype instead of your normal class feats

It's closer to the way D&D 4e did multiclassing. 5e-style multiclassing simply isn't possible in PF2.

But yeah, "multiclass archetypes" are the closest you can get in PF2 to multiclassing. Other non-multiclass archetypes exist too, and these are most similar to D&D 5e's "prestige classes", which exist only in the form of a single Unearthed Arcana (the "Rune Mage") and a few homebrews (Walrock Homebrew's "Vampire" prestige class was especially good). Basically: it's things where you get too many different abilities and have some kind of progression, so it's too much for a simple feat, spell, etc., but not enough for a full class, and probably not something you should be at level 1.