No, archetypes are not subclasses. They're a whole system of character modifications, most of which can be taken by any character as long as they meet the prerequisites. They usually modify some base element of your class (eg the Flexible Spellcaster archetype changes how casters select their spells, use their spell slots, and how many spells they get). There are a subset of archetypes (Class Archetypes) which are locked to specific classes, and which more deeply alter the class's base abilities. The changes can be quite significant. This is where the presteige classes are rearing their heads.

PF1e archetypes are similar to subclasses - many functionally are - but are more about using one class as a base and replacing the parts you don't want with parts of other classes you do want. Each archetype is linked to a specific class (which contributed to PF1e's bloat), but you can stack any that don't affect the same class features, and most classes have equivalent archetypes - for example, nearly all the non-companion classes have an archetype to replace something with an animal companion, most of the casters have a martial archetype and the martials a caster archetype etc. You can still multiclass on top and add archetypes to the new class(es), but they're not quite class agnostic so I guess I can see why you wouldn't like them.

PF2es archetypes/dedication feats are fully class agnostic replacements for PF1e archetypes/3.x prestige classes/multiclassing in general - you take them in place of class feats, and have to take a certain number from the same archetype before you can choose a new one. Medic is very popular because feats like battle medicine and ward medic lets any character replace a dedicated healer. Because of PF2es feat-centric design your initial choice of class gives you quite a loose framework of abilities to choose from, which you can then expand with dedications in more agnostic directions, like healing, duel wielding, archaeology, or becoming a lich.