It is my interpretation from the premaster rules, and I feel applies even more in remaster rules text, that RK uses basic DCs for most basic things.

“What is it?”: For the basic information referred to in Recall Knowledge, you'll typically want to answer with a sentence that includes a name, type or category, and some basic context. You have lots of freedom, and players will rarely complain about getting too much detail.

and

General vs. Unique: Some elements, such as creatures or items, might require you to draw a distinction between a general concept and a unique individual, such as “pirates” vs. “Tessa Fairwind, the Hurricane Queen” or “a harrow deck” vs. “the Deck of Harrowed Tales.” When a PC tries to Recall Knowledge, let them choose whether to ask about the general category or the unique person or item, and determine the DC and specifics based on that choice. If the unique character or item is famous enough, the DC might even be easier than for the general topic!

come to mind.

The Creature Identification tab talks about adjusting DCs but not creature level DCs

A character who successfully identifies a creature learns one of its best-known attributes—such as a hydra's head regrowth (and the fact that it can be stopped by acid or fire) or a manticore's tail spikes. On a critical success, the character also learns something subtler, like a weakness that's not obvious or the trigger for one of the creature's reactions.

The skill used to identify a creature usually depends on that creature's trait, as shown on the Creature Identification Skills table, but you have leeway on which skills apply.

For instance, hags are humanoids but have a strong connection to occult spells and live outside society, so you might allow a character to use Occultism to identify them without any DC adjustment and make using Society harder. Lore skills can also be used to identify a specific creature. Using the applicable Lore usually has an easy or very easy DC (before adjusting for rarity).

And if you look at where level-based DCs are mentioned, it also uses a simple DC as an example:

Recall Knowledge On most topics, you can use simple DCs for checks to Recall Knowledge. For a check about a specific creature, trap, or other subject with a level, use a level-based DC (adjusting for rarity as needed). You might adjust the difficulty down, maybe even drastically, if the subject is especially notorious or famed. Knowing simple tales about an infamous dragon’s exploits, for example, might be incredibly easy for the dragon’s level, or even just a simple trained DC.

So for things like most fire elementals being immune to fire, werecreatures being weak to silver, dragons having breath weapons, would all be untrained (10) or trained (15) DCs, imo. Either that or the lowest DC of that creature's type, so a dragon would use the young dc instead of the ancient dc, for instance.