How accessible are research opportunities, especially for freshmen and sophomores? Are there a lot of underclassmen doing research, or are research positions mostly given to upperclassmen?
By the end of sophomore year mostly everyone who is interested has a research position under a faculty member (for the hard sciences at least). As a freshman it is much harder to find opportunities, being basically impossible the first semester. If you’re really dead-set on starting research in your first year, it’s common for professors to support research opportunities for freshman who have received A+'s in introductory courses they (the professors) have taught.
Some incoming freshmen are offered research opportunities through the Hunter R. Rawlings III Cornell Presidential Research Scholars (RCPRS) program, although it is possible to enter the program later. My daughter was offered this opportunity, but sadly Cornell wound up not being affordable, so she accepted a scholarship package elsewhere (that also included a special research opportunity).