Grad schools are not looking to figure out from generic ECs which teenagers, who mostly have no clear vision of their future, are “well rounded”, engaged, seem to persist in their interests, and whatever other indicators they can’t derive by looking at HS GPA, that those teenagers likely fit into their (undergraduate) college vibe and culture, or fill a particular subset that balances-out their freshman class.
Once you’ve already succeeded in college, this is instead very much like applying to a job (indeed, it might be the equivalent of a full-time job for years).
Experiences specific to the program you are applying to will matter, whether by working, or volunteering in the field, assisting with/conducting research. Professors will invest years with you - so they’ll try to assemble a group of cohorts who have demonstrated commitment to the field. Someone realizing it’s “not for them” down the road would be a huge loss to the program.
I would visit university websites, and research programs that you’d consider applying to and then carefully read all the information from that department/program, as well as admissions, as to what their minimal application requirements are, and what experiences their ideal candidate should bring to the table.
Then see how you can build up your resume, by interning, volunteering, or otherwise engaging with organizations related to your field of interest.
My daughter had been fortunate to have landed a summer internship, a practicum at a practice, and a full-year research internship while in college - all approximately related to the graduate program she was eventually applying to. And, as an actively serving EMT since junior year of high school, and having been named a line officer, she had thousands of hours of immediate patient contact, as well as real-life leadership and training responsibilities.
Given the selectivity of some programs, and the high academic standing of all candidates, I have to assume that those other factors did play a role why she was able to directly land interviews at multiple grad schools, rather than retrying for a few years while working in the field.