It is in tge DMG section about setting DCs, and again it is all over examples of play. Won't keep it up, because it is seriously off topic.Where is it rules as written? I don't see it in the PHB in the Skills section, nor in the DMG section about skills. Those sections largely talking about when to apply proficiency bonuses or not for various uses, not gating skills.
On topic, I gave up on PF2E in the playtest after reading the Skills section.