**NOTE ON INSTALLATION** While you can use Bootstrap with just the CSS file, this will disable many of the features such as the drop down menu. You have to download, install and reference the dependent files, or simply link to a CDN of those files. For example Bootstrap 4 requires JS, popper.js and jQuery.
You can can download those files and include them manually into your site files and reference them. You can also install them via the command line with npm. See current bootstrap homepage for installation instructions.
At it’s core Bootstrap is just a bunch of pre-written CSS classes that you can reference. Browse the examples in the documentation.
Some things to remember:
-12 column grid system
-all items must be in rows
-to prevent things from going full width use the wrapper class
-you can put rows inside of rows
-you can overwrite the default CSS styling by specifying the same class in another CSS file that is referenced AFTER the default bootstrap CSS file. Just like a child theme in wordpress! But the 2nd CSS file with customization must be referenced AFTER the first.
To make sure that menu items are included in the mobile hamburger menu, you have to reference them with an ID. See samples.