Bootstrap looked like a shortcut. I could drop in a class called col-md-6 and get a two-column layout without writing a single line of CSS myself. For about 6 weeks I built things that looked reasonable on screen, and I had no real understanding of what was happening underneath any of it.
Where it fell apart
A client asked me to remove the default gutter spacing between columns on mobile only. I had no idea how to do it. I searched for Bootstrap-specific solutions, found 4 different answers across Stack Overflow threads from different Bootstrap versions, tried all of them, and broke the layout twice. The actual fix required understanding CSS media queries and the box model, which I had skipped entirely by jumping straight into the framework.
Rebuilding the foundation
I spent 3 weeks going back through CSS fundamentals using the MDN Web Docs reference, specifically the sections on the box model, flexbox, and media queries. After that, Bootstrap made complete sense. The framework is just pre-written CSS, and once you understand what CSS does, reading Bootstrap's source or documentation becomes straightforward.
Frameworks are genuinely useful once you know what they are abstracting. Before that point they are a layer of confusion sitting on top of another layer of confusion. If you are starting out, 3 weeks on raw CSS will make every framework you touch afterward significantly easier to work with.