If transferring after freshman year, most schools will still heavily weight an applicant’s high school record and standardized test scores. When applying as a junior transfer, many schools do not consider HS record/test scores.
Bottom line, you do need to check each school’s website to see how they evaluate transfers based on the year they are applying and/or how many college credits the applicant has.
Note that many Ivies take few transfers…the acceptance rate can be less than the incoming frosh acceptance rate. Look at each school’s common data set for transfer information. Many Ivy transfers are recruited athletes or from two year schools with established relationships, such as Deep Springs, or NY community colleges in Cornell’s case.