I've always loved the concept, and yet was always the student that would scoff when lecturers mentioned reviewing. The problem is "active learning" doesn't work outside of small groups at a similar "level" so to speak (as in background knowledge, dedication, and interest).

Very few college classes meet these requirements. When they did, they were amazing. But otherwise, reviewing just makes it nearly impossible to pay attention as the lecturer slowly speaks the material you already know. And then any questions you may have require too much detail to actually answer in the lecture. It's really quite miserable. Why would you do that to yourself?

In a course I teach when the isolation measures started in my country we got 2 weeks in advance to turn everything from in-person classes to virtual "maybe for a month, and then we'll go back" (you can imagine how that turned out).

So during 2020 we moved into a model where we recorded the lectures for students to watch on their own time, and then they'd have a questions-only class (or so they were told). This turned out to get 80-90% of the students to actually view the material before class, and then they'd ask questions about what they didn't understand. On a few difficult topics, we ended up having yet another lecture (but focused on the parts they had trouble following).

On some occasions where they didn't engage with many questions (I think it was the first few classes under this model, which was novel to the students) we the teachers picked up on doing a quick recap and focusing on what we knew beforehand tends to be hard to understand, and engaged them in the class (questions, explain the concepts themselves, etc).

Here's my take: it takes a lot of effort to do this, from recording the material beforehand, to "lying" that the class will be questions-only for clearing up (and we know we'll end up explaining yet again if they don't bring questions), to actively engaging and changing the pace of the lectures/classes.

Because of all this effort, out of 5 different courses I teach, I only managed to pull this in 1, and we're still tweaking a lot of content to make it work better under this model. But we're planning to keep it even after the restrictions are gone (we're still not giving in-person classes at my university, in theory they'll be back for 2022).

Hum, if people are engaging, it's not a lecture. Discussions are much more effective than lectures.

What I've never seen is a comparison between a pure lecture (like it would be on video) and reading a book. Those two fit the same stage on an effective "get pointed to the content, get the raw content, refine it with people and the real world" learning process.

Now, about your question, I have no idea :)

This is what I do when learning math and it works really well.

Lectures alone don't give you a deep understanding and a solid theoretical grasp of the concepts and their manipulations, books alone are very dense and often lack the intuition and human explanations of the concepts. But if you go book then lecture you get a double whammy of thick theory followed by an exposition of the intuition behind it and suddenly everything clicks together.

I imagine it's obvious to many, but I only realized it recently.

I think for some subjects and some teachers, swapping homework time and lecture time can work. For example, if you are learning calculus, it may be more efficient for the teacher to assign reading a section of a textbook as homework and then in the classroom work through a bunch of problems and proofs using the homework material.

If they worked through the material prior to the lecture, perhaps the lectures were spurious for them, and the credit for the high grades goes to the individual work.

Credit for high grades always goes to individual work.

I like to think my lectures are good, but you can't learn material without doing the work yourself.

As for being spurious? They didn't have to show up for my lectures--they did anyway. I have never made that a condition for any of my classes.

Also, confirming what you wrote: students who excelled at my courses, usually took some courses, like online, before that. Those who were great at maths in the university, said their parents were mathematicians, and they were exposed to advanced maths, like quadratic or trigonometric equations already at the age of 7-9.

I always thought lectures aren't for teaching, they are basically just some more detail on the syllabus. Basically it's the prof saying "you need to know this proof, I'll skim over it fast and you can figure it out in your own time".

I also taught at commercial courses and in a university, trying to apply active learning, but it didn't go well, so I haven't an answer to this problem either.

That requires the material to be available in advance, which often isn't the case.

Every class I have ever taught required a syllabus and text/reading assignments to be available--generally the entire semester is laid out at the first class.