I have repeatedly come across the comment that experienced writers are able to put their topic sentence anywhere in a paragraph, but I wonder what that looks like and how they are able to do this. How would one go about accomplishing that? What would such a paragraph look like?
Some paragraphs do have a "topic sentence" right at the beginning. This tells you what you are about to read. Then the rest of the paragraph expands or explains that general idea. There's nothing wrong with that as a structure.
There are many other possible structures used in the world of writing, however. One commonly used structure in essays is the funnel paragraph. A funnel paragraph starts out very general and becomes increasingly more specific. You can thus write a paragraph where the topic sentence comes at the very end.
Truthfully speaking, there are a lot of "rules" that you learn when first writing that help you understand structure, or that keep you from being too unfocused, but that can be discarded as you gain proficiency. For example, consider starting a paragraph with some general context. The actual topic sentence that best summarizes your paragraph can be placed right in the middle. You can then follow it up with some additional info, such as the fact that a paragraph can still be coherent, even if there isn't any one single unambiguous topic sentence.
Personally, I would regard this more as a critique of the notion of a topic sentence than as and evidence of skilled writing. The theory of the topic sentence is part of a theory of paragraph design that really doesn't hold much water when you look at the practice of real writers. So maybe the fact that the putative topic sentence can, in fact, occur anywhere in the paragraph is just a sign that the notion of a topic sentence as an analytical category is not very sound (and that therefore there is no problem to solve here).
That said, you could look at the idea of the topic sentence as a way of teaching very basic writing techniques to third graders. Maybe it works as a way to help them organize their thought and write more coherently. In other words, you could look at the idea as training wheels for writers. But the mystery is not that adults ride without training wheels. That is just what normal bike riding is.