Leads?

I'm reading book 2, and in Chapter 3 players can find leads that send them to specific places where they find the specific people who can help them, but it feels unclear what help some of those people give, or why the player would know that, and why the player would know that's a lead, and how they'd find out. For example, Jhebek is listed as "Uncover Narseigus's magical obsession", but it's never really written other than that. There's no place in the book (from my 1 read's memory) where it's said "Narseigus has an obsession with magical objects, and is doing X for it", or what Jhebek would actually know about it. It's in the campaign summary in book 1, sorta, but not really elsewhere. The help Jhebek offers has nothing to do with it, it's an act of service, and it's not like Quolko is going to be in any mood to advise the players after they beat him. I'm also not sure how I'd lead players to it. Do I just tell them "gather information says that you'll find the next hint at the forge district"?
Sorry if this is too much of a "new DM" ask.
There is an in-built explanation as to why players will hit up Durruzzin, and at the very least Bronwyl will find the players herself if they don't think to ask "so who are Narseigus's rivals?", but the other clues feel more difficult to hit, and Jhebek feels like a kinda extreme example of the quest being unrelated to the solution, which isn't really included.

I think you are right that this section in particular is confusing. As far as I can make out, the magical obsession / statue is a reference to the cave worm statue found in his apartment (room J7). The problem is that this is never actually stated in the The Infinite Quarry.

As for picking up the lead, under the heading "The Infinite Quarry", it says:

When they gain the Strange Artwork lead, the PCs learn that Narseigus commissioned a mysterious statue from a sculpting workshop in the Forge District.

So I think the idea is that the PCs first spend some time to Gather Information about Narseigus. No DC is set for this, so presumably it is meant to auto-succeed, but you might set an appropriate DC if you want them to have a chance of failure. On a success, tell the party that they hear a rumour that "Narseigus commissioned a mysterious statue from a sculpting workshop in the Forge District." When they go there, they might learn about the cave worm statue from Tralginfad the Lesser. Regardless of how that interaction pans out, Jhebek signals the PCs shortly after they speak to Tralginfad, offering to tell them what they want to know, in exchange for them securing his freedom from Quolko.

The lead about military movements could be given to a PC who Gathers Information without a specific goal in mind, or about 'recent activity in the city', or something that you consider relevant enough.