Use Media Queries. Media Queries sets conditions for CSS to be used in specific scenarios. If these conditions are approved, that is, if the device fits all the conditions set out in your Media Querie, the CSS will be applied.
// Extra small devices (portrait phones, less than 576px)
@media (max-width: 575.98px) { ... }
// Small devices (landscape phones, 576px and up)
@media (min-width: 576px) and (max-width: 767.98px) { ... }
// Medium devices (tablets, 768px and up)
@media (min-width: 768px) and (max-width: 991.98px) { ... }
// Large devices (desktops, 992px and up)
@media (min-width: 992px) and (max-width: 1199.98px) { ... }
// Extra large devices (large desktops, 1200px and up)
@media (min-width: 1200px) { ... }
With Bootstrap, if you already knew, you could implement it to the site, but without knowledge, I suggest redoing the whole structure of the site and adding it. Without Bootstrap, you can make yourself resposive using CSS techniques such as flexbox and media queries. I advise you to learn these CSS techniques first and then use CSS frameworks like Bootstrap, Bulma, Materialize, etc... to know what they do behind the scenes. There’s plenty of this kind of stuff on Google and Youtube.
– LeAndrade
Completing the answer of Leandrade. Imagine that you are a tailor, then you draw, cut and sew a whole suit, then the client wears the suit and says that it was small... Look at the size of the M... Now imagine that each page you made is a different suit and you made 5 or 6! Something you will be able to enjoy, but it would probably be better to start again from 0 because the responsiveness involves the structural part of HTML, vc needs a Grid etc. A thorough study before starting to redo. Study a lot Flexbox or Grid Layout. Then you can think about using a framework...
– hugocsl