WordPress is a really powerful platform. Making a responsive theme and integrating it is pretty easy once you know the basics of theming. Responsive images, on the other hand, are not something WordPress deals with out of the box. You could go down the route of manually generating each image size, then in the HTML editor you could manually type the image tag, srcset attributes and each image. This will not only take time, but could be an issue for any admin users that are not HTML-literate.
Thankfully there is a way to make WordPress do all the heavy lifting. It can generate all the image sizes from a single image upload then, via a plugin, implement the image tag and