WTF? Please read my actual play posts, to which I've provided links, and then get back to me.
I find reading actual play posts to be quite boring, TBH and not actually useful in determining how a game is actually played, since they rarely go into details about how the mechanics work. For instance, there's nothing in the pdf you linked that describes Wises. Maybe it's listed in the actual book, but the doc is only 74 pages long. (BW does, apparently, assume orcs are hateful, which is a point against the game IMO).
And also TBH, you're not doing a very good job selling the game, which doesn't bode well for the actual play post being any more interesting.
By "PCs" do you mean players? Otherwise what you ask makes no sense to me.
No, I mean PCs. The players are, presumably, sitting around a table or in front of a computer in a video call. They are
Playing Characters in the game. These are the PCs.
The PCs are entering a tower. Describe it.
Or are you saying that, in BW, nothing is described unless the players first establish it? Because wow, that's boring.
Assuming that that is what you mean, as I've already stated - multiple times, I believe - the GM does the framing in ENworld. But suppose that it had already been established, say via a Wises check, that Evard's entrance hall is painted all in crimson red, then of course the GM would include that in their description.
I'm gathering, through some googling, that Wises means "skill" or "knowledge" (which, along with Artha, rather annoyingly means that BW is one of those games that feels the need to rename everything in order to be cool and different). So... what sort of skill check is needed to see the color red in this game?
Burning Wheel's approach to resolution, and its corresponding approach to PC build, lends itself to a relatively "gritty" attention to detail. (Compared to, say, Marvel Heroic RP, which relies on broad-brush and trope-y Scene Distinctions.)
I am missing what connection any of this has to deciding in advance of play where spellbooks might be found.
Really? You don't understand how the game handles description may have any effect on how the GM describes things?
Do you really think that no one can imagine a cave or castle whose inhabitant is away from home, unless a rulebook writes in a % in lair chance?
The point of the % in lair chance is to establish a gameplay process for managing the introduction of content into the shared fiction.
Burning Wheel uses a very different sort of process. I've described it at quite a bit of length in various posts in this thread, many of them in reply to you.
At this point, I have to assume you are deliberately misinterpreting what I'm saying.
No, clearly people can imagine creatures being away from home without the need for a roll. What I said is that even back in the earliest days of RPGs, people knew that the game wasn't limited to just what the PCs saw.