I’ve been playing guitar for 15-ish years but never really learned scales.

As the title says, I’ve been playing guitar for 15-ish years and I’m okay but I’m mostly self taught (I had a guitar teacher for 6 months when I started at age 11) so I have some glaring shortcomings that i want to improve upon. For instance, I only ever learned one scale and I can feel that lack of knowledge/understanding almost every time I play. I’m looking to learn different scales and better understand music theory. My friends who read and write music tell me that some of the music I listen to does interesting things with music theory (but I don’t fully understand what this statement even means). Ultimately my question is what are some resources, or exercises, or anything for learning and better understanding scales and broader music theory? Any advice and/or insight is appreciated.