Learn French: Le supermarché est à côté de la boulangerie.

You would ask Où est le supermarché ? (Where is the supermarket?). The word où means 'where', and it is placed at the beginning of this simple question to ask for a location.

Why are there three words (à côté de) used just to say 'next to'?

The phrase à côté de literally translates to 'at the side of'. It is a fixed expression in French, so you must always use these three words together to say that something is next to something else.

Why is it de la boulangerie instead of just de boulangerie?

In French, you almost always need an article like 'the' or 'a' before a noun. Because boulangerie (bakery) is feminine, we use the feminine word for 'the', which is la. Combined with the de from the phrase à côté de, you get de la boulangerie.

If the second location were a masculine place, like a restaurant (le restaurant), would it be de le restaurant?

No, and this is an important rule to remember! In French, the words de and le always merge to become du. So, 'next to the restaurant' becomes à côté du restaurant. Since boulangerie is feminine, de and la do not merge and remain separate.