Bootstrap4 site design - what's the better style approach?

Is it a better to maintain a separate CSS file in conjunction with the Bootstrap html files? For example, the Gloria’s Flowers exercise in Learn Bootstrap course is self-contained, while the personal website that’s developed in the Make a Website course utilizes a CSS file with the Bootstrap pages. Thanks, Bob

bootstrap html files? What do you mean by that? Understanding what bootstrap is and why you should use it, should be the first step

bootstrap is according to there own website a front-end component library

there is more to bootstrap then just CSS, but lets focus on the CSS for now. Using bootstrap means we have pre-made CSS which enables developers to more rapidly style the site and make it mobile friendly easier and faster.

So bootstrap isn’t mandatory, and maybe not everything you need is included in bootstrap, so you can also add your own CSS file(s).