There are a few different materials that lower receivers can be made out of, but the most common material is aluminum. There are also polymer and steel lower receivers, but aluminum is by far the most popular choice. Aluminum lower receivers offer a good balance of strength and weight, and they’re also relatively inexpensive.
Polymer lower receivers are becoming more popular in recent years, as they’re even lighter than aluminum lowers. They’re also generally less expensive than aluminum lowers, although there may be some tradeoffs in terms of strength and durability. Steel lower receivers exist as well, but they’re relatively rare and typically quite heavy (and thus not particularly popular for use in AR-style rifles).