What to learn first: bootstrap or website responsiveness?

Question

Bootstrap

My knowledge of the web is limited to HTML, CSS and a couple of made-up sites. I decided to learn how to make adaptability for sites, I watch different videos on YouTube, but almost all videos use some kind of framework, ala bootstrap, grid, etc. Tell me what I need to learn at this stage? Maybe I misunderstood something, and is it possible to make an adaptive without any frameworks? And if you need a framework, what is better to learn, grid, bootstrap or something else? (please answer in the most understandable language, I'm a teapot)