Learn Italian: Abbiamo scelto lo stesso ristorante.

When stesso is placed before a noun, it means "the same" (pointing to exact identity). If we placed it after the noun (il ristorante stesso), it would change the meaning to "the restaurant itself" (for instance, if you were emphasizing the physical building rather than the food).

Why is the article lo used here instead of il, since we normally say il ristorante?

The definite article must always agree with the word immediately following it. Because stesso starts with an s + consonant (st-), it requires the article lo. If you removed stesso, the article would revert to il (il ristorante).

Could I use medesimo instead of stesso in this sentence?

Absolutely. You could say Abbiamo scelto il medesimo ristorante. Medesimo is a perfect synonym for stesso when used before a noun to mean "the same", though it sounds slightly more formal. Notice that the article changes back to il because medesimo starts with an 'm'.

How would this sentence change if we chose "the same restaurants" in the plural?