What parts of the chicken are tenderloins made from?

They come from the pectoral muscle, the same muscle that forms the chicken breast. However, they are specifically from a smaller, more tender part of the breast called the "teres major".

This muscle is located on the inside of the breast, near the wing, and is often referred to as the "tenderloin" due to its unique tenderness.

It's important to note that not all chickens have a well-defined teres major muscle, which is why sometimes "chicken tenderloins" can be made from other parts of the breast or even from smaller pieces of the breast that are mechanically separated.