For DS, he submitted his PSAT during Junior year as part of his SLE application (pre-candidate file). He got into 2021 virtual SLE. So USMA already had a standardized test on file when DS started the SA application.
Once his actual SA application opened, the portal indicated submission of school transcripts at specified due dates. The transcript is directly submitted by the school counselor or whoever's in charge of transcripts in your school.
If you think you can manage the load well and "As" it, then you should challenge yourself. DS had barely any high school subjects left his senior year because he took them in advance. So he took classes in community college, volunteered in the community, had one semester of internship, and participated in group sports. You can always find something worthwhile to fill your schedule if you really want to - some seniors have work experience and get great leadership recommendations from their managers.