Weird but specific art library question

"Do art libraries, and material libraries reload themselves upon each fresh launch of LB? I would assume so."
Yes. The libraries are loaded upon startup or “Load”.

"Do they also reload themselves if you open a saved project or are those libraries cached within that saved file?"
I’m interpreting this to mean that you’re asking if the art libraries are saved with the project file. If so, the answer is no. The Art Library is independent of the project file. In the case of material libraries the project file can have a reference to the Material Library for a linked material but the library itself is not stored with the project file.