In order to build modern web apps, you'll need HTML, javascript (JS) and CSS.
Bootstrap is not an alternative to React. They're totally different. But you can use Bootstrap inside React for styling
Note: Bootstrap contains some JS code for modals, popovers etc. But it's not recommended to use them with React, because they're based around JQuery.
I would suggest you learn JS and CSS basics first, then try these frameworks, you'll get the idea