Chance a rising senior for Yale (or Harvard?) REA + my reaches [3.77 GPA, 1560 SAT, <$10k, economics+CS/math/datasci]

No actually the admissions rate does not remain the same for all applicants. Not all applicants will be equally qualified. Some students apply and are not qualified which lowers the overall acceptance rate. Others apply and while they may be well-qualified, they are less competitive than other applicants. That is why it is best for the student to consult the school’s college counselors as the counselors are better equipped to assess how competive the student is in relation to their peers.