<p>Two things:</p>
<ol>
<li><p>While there are classes with 200+ people, there aren’t a whole lot of them. There are plenty of classes with less than 50 people, many with less than 20. Most intro courses are probably 60-80 people. In those cases, yes, there will be sections, and yes, they will probably be led by TAs. Usually the TAs are grad students, but sometimes they are undergrads. It doesn’t mean they are bad/incompetent. I’ve had TAs whose abilities supersede the professor’s.</p></li>
<li><p>Different professors will handle sections differently. I took an excellent course freshman year where we had lecture for an hour and then section for the last 30 minutes of the class. TAs facilitated the sections, and the professor moved between them, observing and offering her insights.</p></li>
</ol>
<p>That being said, I didn’t have any classes with more than 20 students this year, and my department is pretty big.</p>