Quote Originally Posted by Schadenfreuda View Post
What rules, if any, govern how monsters with innate spellcasting such as dragons or Solars use spells with XP components? If a Solar casts Miracle, what resource is expended? Do they have some daily or weekly XP budget, or do they take negative levels after so many uses or something to that effect?
They follow the same rules as casters with class levels. NPCs have as much or as little spare XP as the DM decides they have.

And for that matter, do NPCs in general have rules governing magic item creation and where they get the XP for that? I can't imagine every enchanter out there just happens to also be a mercenary adventurer going out and raiding dungeons all the time to keep up their business; and if they actually all are, I shudder to think of the ecological consequences for the monster species they must hunt.
D&D is a roleplaying game, not a simulation. The rules are an abstraction for the players to interact with the game, not how the game world actually works (unless you're playing in a OOTS-like campaign at least).
Joe Commoner can't become level 8 in a few weeks by going out to kill a CR 1 animal every day (a PC could) and NPC crafters don't need to go murderhoboing to collect the souls of their enemies as crafting fuel. Not by default at least.