There are 8 Ivy League colleges, and although they are highly competitive, they aren’t necessarily for everyone. https://insights.collegeconfidential.com/what-are-ivy-league-colleges
Ivies? Harvard, Yale, Princeton, Columbia are commonly considered tier 1 Ivies, Penn, Cornell, Brown, Dartmouth as tier 2.
Better or more selective? It depends upon which colleges are they being compared to. If you are comparing them with other T20 schools like MIT, Stanford, UChicago, CIT, Johns Hopkins, Duke, Rice, Vanderbilt, Amherst, Williams than not necessarily.
If you are comparing against your generic state or privates with >40% acceptance rate and less than <80% 4 year graduation rate then yes.
Why they are so popular? Prestige, marketing, history, per student endowment, generous need based aid, successful alumni, location, faculty, student wealth, selectivity, lovely campuses etc.