Hello! I think "built jock" refers to his physical complexion. A jock is someone who plays sports, particularly at school, so in this case, a built jock is a really strong boy who plays sports at school. The "varsity sports circuit" is the name for the main teams at the school in different sports. I hope to have helped you!

"Built jock" is a phrase with two slang terms in it, so don't feel bad about not knowing the meaning. "Built" in this context means "very muscular/having a high level of fitness" (similar slang terms include "cut," "ripped," and "jacked"). "Jock" is a slang term for an athlete/sportsman, especially in the context of high school or university. It also generally has an association of the person being somewhat of a bully and not highly intelligent due to how this group was often portrayed in movies or on TV (Biff in the classic film "Back to the Future" is a good example). "Varsity" refers to the main team that represents a high school in sports, and it applies to each sport. I.e. varsity football, varsity baseball. Many schools will also have a lower level team known as the "junior varsity." And a circuit means more than one, a group or collection of things. So putting it all together, the boy in question is a high school student, who is very muscular, and plays on many of the top level sports teams at his school.

Saying that a school has a ‘sports circuit’ is unnatural. Perhaps it’s to show that the speaker himself isn’t knowledgeable about sports. Her boyfriend’s this built jock who stars/plays on several varsity teams at school.