McDonald's is an example of this I always hear brought up. Your job can be making the burgers, but this consists of putting a pre-made patty in a microwave and putting it in a bun with toppings. Since you're so disconnected from the actual processing that makes the patties you don't actually learn how to cook or make a burger no matter how long you work there.
Likewise, if you're working the job of 3 people but getting paid for 1 you'll probably never be able to deep dive into learning more. You might have some on-the-fly experience picking up someone else's slack or pitching in at another department, but doing so doesn't make you an expert. Just as working in fast food prep won't make you a chef.