Learning Bootstrap 3 or 4 should take you very little time, I would suggest you don't learn either of them, but instead take the time to learn CSS (I am of course assuming you are not already a CSS wiz).
You can look at the source of Bootstrap to see how they accomplished certain things if you'd like, but if you're doing anything more than prototyping (and even then), I feel there is very little benefit to using Bootstrap these days.
Once I was told to ignore Bootstrap and just create my css myself (using Sass or CSS Modules) I find I'm making the same recommendations to others. It doesn't take long and you'll have a much better idea of what is happening on your page.
Your html and css should end up being much smaller as well.
You can look at the source of Bootstrap to see how they accomplished certain things if you'd like, but if you're doing anything more than prototyping (and even then), I feel there is very little benefit to using Bootstrap these days.
Once I was told to ignore Bootstrap and just create my css myself (using Sass or CSS Modules) I find I'm making the same recommendations to others. It doesn't take long and you'll have a much better idea of what is happening on your page.
Your html and css should end up being much smaller as well.