I wrote my first bootstrap based website about a month ago. Here is how it typically goes. You look around the web for a bootstrap template you want to use. Some are free and some you have to pay for. Once you find one you like, you download it and paste your text and images into the template. I think this is how most developers use it.
The templates are html that use the bootstrap base css and possibly add more css of their own on top. As far as I know the bootstrap base css is a collection of libraries of css combined with javascript that are well organized and have most of the stuff that a designer needs. So, this all is really just a well organized templating and piggy backing on others accomplishments. Of course whenever you do this you are limited to what has already been built for you, unless you are a well seasoned web designer, which is a specialty in itself. If the template is well done and you don't mess up its code it should nicely size itself to all devices.
Make sure you
test the template to see that it does everything you want, before spending a bunch of time substituting in your own content.