I will discuss recent work on N=4 SYM with Gaiotto-Witten boundaries, defects and interfaces. This broad class of theories with its high degree of supersymmetry and rich defect dynamics is a natural laboratory for exploring the physics of boundaries and defects, and it contains boundary conformal field theories that underlie string theory realizations of double holography. We will discuss a tight interplay between the holographic duals of these theories and the matrix models arising from supersymmetric localization, defect one-point functions, and large-charge operators.