There's not a single solid answer along the lines of web standards as there's really not a web standard for how to DO responsive design. ... welcome to the land of front end development, where everything has 20 answers and none answers your question exactly... ;) Everyone will now probably point you towards more general CSS courses, and now I will too ;)
You've gotten a taste of being responsive with Bootstrap, with columns collapsing on top of each other at various breakpoints. Honestly IMO if you don't want to go down the rabbit hole, a good CSS framework like Bootstrap (or others) will fit the needs of most folks as far as responsive layout goes.
Otherwise responsive design isn't just layout changing with size, it can also be loading different images (smaller sized images if the screen is small) and etc. Then you're well into various CSS topics.
I suggest looking into more general CSS topics / courses that cover responsive design.
I really like https://frontendmasters.com/, they might not have a class just for what you're looking for, but there are some that cover much of what you're asking: