Not sure exactly what you are asking. You can see retention rate (of last year’s frosh class only) in B22 of the CDS, this number is probably mostly transfers (because schools get to adjust the original number for things like death, disability, etc.). We don’t know to what schools those transfers went though, or really if a given student just dropped out.
Here is Harvard’s CDS, showing a 98% retention rate of 2017 entering class into 2018 school year (their second year), so about 34 (1,687*.02) students transferred or dropped out. https://oir.harvard.edu/files/huoir/files/harvard_cds_2018-19.pdf There would be transfers/dropouts after soph year too, fewer after junior year…I am not sure where one can see those, maybe it’s in IPeds.
Ivies take transfers in from a number of places—two year schools (e.g., Deep Springs, certain community colleges), the armed forces/veterans, athletes and of course, other four year schools.