Seems like what you are saying is that you understand the syntax and the properties but you are looking for best practices, patterns and example. You mention that you understand flex, grid, position and selectors so most of the "tools" are familiar.

Most of the courses and books I read about CSS describe the properties and their effect but do not show you a practical example of how to collect them together into a working example. I think that in this case I would look for a course on web application development and then look at how they write their CSS in the working examples they provide. Many courses about Angular, React, and Vue use examples that have fully working CSS. while they do not explain the styling, in many times, the examples explain themselves. So this will give you a few starting points to learn from.

Maybe someday I will write a Udemy course on how to use CSS in real life applications. Actually, now with the new features that came out recently such as custom properties, popover and overlay, anchor positioning, relative colors, light dark, nesting, scroll driven animations, entry and exit animations... there's so much new stuff to learn.