they're really efficient and cheap in my experience. they sell no-bullshit products. they don't spend much money on advertisement so they don't have to add that money to their expenses and therefore prices.

supermarkets make most money with highly-processed foods. every processing step adds another middleman that wants a slice of the price and that already increases the price, so you pay $5 instead of $2 for the raw products, and then the supermarket thinks "well, let's add a 10% profit margin" and makes that price $5.50 instead of $2.20, thereby further increasing the difference in prices. aldi mostly sells basic products that are not so heavily processed (at least where i live) so there's not much extra costs there.