I just looked up Belgian fries, and they do it right: double fried in tallow.
At one time McDonalds served excellent French fries. They used fresh potatoes and fried them in tallow.
I do them in olive oil. Incredible, especially with Sauce Andalouse.
That double frying was why McDonald’s fries were good back in the day, even after they switched to vegetable oil (just not as good as when they used tallow). When I worked there 50 years ago, we made fries out of fresh potatoes that were peeled, sliced, washed, blanched at 280 degrees, and cooled before the final frying at (I think) 325 degrees. This allowed the interior to cook before the outside is finished to crispness. The Belgians apparently invented two-stage deep-frying of potatoes a long time ago.
If you try to fry potatoes from raw to done in one step (as many restaurants do), the outside will get overdone before the inside is completely cooked. They might taste OK for the first minute or two out of the fryer, but pretty soon they’re not so good.
I don’t know if the frozen French fries that McDonald’s uses today are partly cooked before freezing, but I would suspect so.