New teacher/new sub (dumb question)

I'm diving into teacher as a second career and currently waiting to finish my Masters and student teach in the fall. I just applied and got a job as a sub in a nearby school district. I've seen "practice lesson plans" for my teaching classes and given mock presentations in front of my cohort (where everyone is checked out or waiting their turn). I guess my real question is what is going to happen when I sub? At the training, they just went over the steps of getting there early and checking in w the principal's secretary not what to do when 20+ kids are staring at me waiting for some sort of direction.

If I pick up an Art class or a Math/Science class, am I literally teaching Art or Science lesson plan, even if that's not my area of expertise? How often is it that compared to having kids do a worksheet or something? I assume I'll be less stressed once I do this multiple times. TIA.