I recently watched a great hour-long documentary on outsider art, called Turning The Art World Inside Out. The term "outsider artist" has never sat completely right with me, because it feels sort of exclusionary and othering.
Wikipedia defines Outsider Art as:
art by self-taught or naïve art makers. Typically, those labeled as outsider artists have little or no contact with the mainstream art world or art institutions. In many cases, their work is discovered only after their deaths. Often, outsider art illustrates extreme mental states, unconventional ideas, or elaborate fantasy worlds.
That said, Turning The Art World Inside Out offers insight into the practices of many so-called "outsider artists" and their unique visions. One of my favorite artists from the documentary is Ionel Talpazan (1955-2015), who made magical, colorful UFO paintings. He's interviewed in his apartment/studio and talks about his life, art, and thoughts about UFOs.
You can watch the documentary on YouTube: