I’m a little confused how this differs from standard constraint optimization.
That's how SNOPT, IPOPT, presumably KNITRO and nonlinear programming optimizers work.
Yes MDAO is "just" constrained nonlinear optimization.
You come up with a model for your thing, which often involves multiple "disciplines" like mass, propulsion, aerodynamics, loads, trajectory/equations of motion, and then usually use some framework to calculate the constraints values and objective value and their gradients