>And how do you “choose” a reputable shop without knowing where the parts come from?
You do it either based on brand, word-of-mouth, recommendations, online reviews, or a number of other ways people have been using to find reputable service centers for decades. People who came before us fought hard and we have established a precedent for repair in many many industries so that companies like Apple are not able to rip us off by denying right to repair, and denying legitimate repair shops the ability to purchase legitimate parts.