From what I have seen, and from what my kids have experienced, the more selective colleges and universities place much more emphasis on critical analysis and writing, and discussions, and provide more feedback to students on their writing and analysis. The less selective schools are more likely to use multiple choice tests, or teach straight from the textbook - particularly if they are using part-time adjuncts who are told to follow a standard curriculum.
It has often been said that you learn more from fellow students than from your professors. That is particularly true with discussion classes, or group projects, or classes where there are study groups. Employers value employees who know how to work in small groups, which is more likely to happen at a selective college.
The more selective colleges are more likely to have more motivated and well-prepared students, which allows a professor to be much more demanding of the students.
(My Dad was a college prof at a non-selective college, and he greatly preferred teaching the evening classes of adults who had gone back to the college, because they were motivated, vs. the daytime classes of younger students who in some cases were just killing time.)
Many of the selective universities will have large classes in the first two years. However, they will often have highly qualified grad students leading discussions. Some grad students may be teaching freshman math or writing classes. These grad students may be a few steps away from earning a doctorate and seeking a full-time college teaching job. (However, I’ve also heard of some less motivated grad students, particularly if they are only interested in research and not getting a teaching position).
The more selective colleges are also likely to have students from a much wider geographic area and from other countries, which provides another dimension to learning.
My son and I each had the opportunity to graduate from a selective university. I only had mediocre profs in 2 of my classes, and I hadn’t heard my son complain about any of his profs. Meanwhile, my daughter attended a much less selective college and had several mediocre profs outside of her major. Fortunately, that college is excellent for her major.