The two functions mkYesodData and mkYesodDispatch in the Yesod framework are supposed to separate the handler definition and the dispatch process. Though by some miracle (to me), templates use this interesting function "resourcesApp":
mkYesodDispatch "App" resourcesApp
The only mention of this function I have found in hoogle is in the Hledger package. And it is not a yesod dependency.
In the school of Haskell by this link they give an explanation that resourcesApp is "generated" by mkYesodData, although it still does not work for my side. https://www.schoolofhaskell.com/school/advanced-haskell/building-a-file-hosting-service-in-yesod/part%202
What is the reason for this?
There's some Template Haskell (TH) going on under the hood in Yesod, and I think this is what's confusing you. Template Haskell can be confusing when searching in documentation because it produces values at compile-time for use at runtime that aren't there before the code is compiled. resourcesApp is just one of these values.
In the code you reference, the author describes that you must have another module (which he calls Foundation) in which you have invoked mkYesodData. Indeed, without this other module, the code in the Dispatch module won't work. Strangely, it's not until (Part 4)[https://www.schoolofhaskell.com/school/advanced-haskell/building-a-file-hosting-service-in-yesod/part%204] that he seems to define the Foundation module, but you can see that there is a line:
mkYesodData "App" $(parseRoutesFile "config/routes")
That may not look like it defines a value called resourcesApp, but sure enough, it does.
In short, you should be able to get your code working by just finishing the entire tutorial and running the code altogether.
In case you're wondeering, a call to mkYesodData takes a String and then literally generates code that defines a value names resources**** where the **** is the string you passed. In this case, that would be a value resourcesApp, but in someone else's Yesod project, it could be resourcesFoo. Furthermore, since this resourcesFoo value isn't concretely in the code, projects that use Yesod typically wouldn't have it show up in their export lists or haddock documentation. It's actually very strange that you found even one hit for resourcesApp on hoogle at all, but upon closer examination, it kind of makes sense: Hledger seems to be some sort of extended interface around yesod, so they pre-generated the TH values so that they would be easily accessible to users.
As another note, TH has some restrictions in its use. For one, you typically need to perform the TH invocations ("splices" as they're typically called) in a separate module than the one you use the generated values. This is probably why the author has you create a separate Foundation module rather than just putting the line mkYesodData ... in the Dispatch module.