Excellent Category Theory lectures by retired Prof Steven Roman from Uni. California: he used pen and A4 – paper with iPhone camera. Simple & good. (Only lighting could be brighter.)
Category Theory is one level higher abstraction, above the Abstract Algebra (Group, Ring, Field, Vector Space, Set…). It is the “Math of Math”, to make difficult math easy by studying the ‘Relationship’ (or Morphism, drawn as Arrows).
Lecture 1: study five topics
◇ Natural Transformatiom
◇ Universal Ptoperty