This paper reports the findings of a qualitative intrinsic case study exploring how a STEM high school internship program with six tracks in Baltimore, MD (USA) promotes inclusive excellence in STEM careers; orients high school students to participate in their social and professional communities in college; and provides students with immersive STEM experiences that reinforce their confidence and academic preparation. The study was guided by two specific research questions: 1) How does this STEM internship program shape high school students' perception of STEM careers? and 2) What are the features of these STEM internships that most contribute to high school students' formation of their early career trajectories? The data came from semi-structured interviews with 11 high school interns participating in these STEM internship tracks during June-August 2024. The data were analyzed using reflexive thematic analysis. Three themes were identified: 1) these STEM internships help participants "connect the dots" between their prior experiences, interests, and skills to their career trajectories; 2) these STEM internships help the interns find their voice and use their resources with confidence and agency; and, 3) these STEM internships help the interns learn how STEM "really works" in practice.