Check out Matt Bowen's Football 101 stuff from when he was with Bleacher Report:
https://bleacherreport.com/articles/2007958-nfl-101-the-basics-of-the-3-4-defensive-front
He's really good at explaining everything.
There are also some books that have been recommended quite a few times on this sub, like this one:
https://www.amazon.com/Take-Your-Eye-Off-Ball/dp/1600783910
Take Your Eye Off the Ball: How to Watch Football by Knowing Where to Look https://www.amazon.ca/dp/1600783910/ref=cm_sw_r_cp_apa_i_Ese5Cb4H8GXDW
Check out Matt Bowen's Football 101 stuff from when he was with Bleacher Report:
https://bleacherreport.com/articles/2007958-nfl-101-the-basics-of-the-3-4-defensive-front
He's really good at explaining everything.
There are also some books that have been recommended quite a few times on this sub, like this one:
https://www.amazon.com/Take-Your-Eye-Off-Ball/dp/1600783910
Take Your Eye Off the Ball: How to Watch Football by Knowing Where to Look https://www.amazon.ca/dp/1600783910/ref=cm_sw_r_cp_apa_i_Ese5Cb4H8GXDW