They are the same thing.
Bootstrap just conveniently did all the styling for you in a class, which is nice, but when you get down to quirkier specifications like wanting special animations, or you want you nav link in parallelograms instead of rectangles. The customizations add up and eventually it can be as tedious as rolling your own CSS.
For the sake of ease, if you just want a nice looking navbar, bootstrap is excellent. If you want to learn exactly how the styling works like how the element laid out, how it responds to change in screen size, why does the toggle work the way it does. You should maybe implement one on your own. It is not the most complicated thing in the world and a good way to test if you can apply the CSS you’ve learned