If you go into Settings - CSS - you can add Bootstrap as an external css via the “Quick-add” options.
Then add whatever css you like to the css editor.
I don’t use bootstrap, but I would think that the purpose of those classes is described in their documentation. If it’s anything like typical CSS, then how it renders depends somewhat on context, so it might be difficult to predict outside of any context. Just guessing about that.
Well there is no html to apply the css to.
You would have to write some html with whatever bootstrap classes or classes of your own defined in the css section.
I just have a hard time understanding the idea of seeing the results of code, without actually writing any code.
That is usually the easiest way to experiment with code. Codepen can be handy as you can include those frameworks, without actually having them. I mainly use it just to share things here.
Dev tools are a good way to tinker with code without committing to file, great for debugging.