Swagger UI 3.x+ has the plugin system that lets you add or modify UI elements. Some documentation on plugins can be found at:
Plugins
What's the intended way of customizing SwaggerUI 3.x?

and there are some examples here: Plugin catalog.

There is no need to recompile Swagger UI to use plugins, you can actually define the plugins directly in the index.html page. To add or change UI elements, you need a plugin that uses wrapComponents and React.createElement to build the desired DOM structure. (See also React Without JSX.)

For the custom plugins to have effect, they must be added to the plugins list in the SwaggerUIBundle constructor.

Example

Here is a sample plugin that adds custom <h3> headers above and below the API title:

// index.html

<script>
window.onload = function() {

  // Custom plugin that adds extra stuff
  const MyCustomPlugin = function() {
    return {
      wrapComponents: {
        // add text above InfoContainer - same effect as above title
        InfoContainer: (Original, { React }) => (props) => {
          return React.createElement("div", null,
            React.createElement("h3", null, "I'm above the InfoContainer"),
            React.createElement(Original, props)
          )
        },

        // and/or add text above API description
        InfoUrl: (Original, { React }) => (props) => {
          return React.createElement("div", null,
            React.createElement(Original, props),
            React.createElement("h3", null, "I'm above the API description")
          )
        }
      }
    }
  }


  const ui = SwaggerUIBundle({
    url: "http://petstore.swagger.io/v2/swagger.json",
    dom_id: '#swagger-ui',
    ...
    plugins: [
      MyCustomPlugin,       // <------ add your custom plugin here
      SwaggerUIBundle.plugins.DownloadUrl
    ],
    ...

The result looks like this:

As Helen alludes to, the answer to your question depends on your setup. However, I would hazard to guess that you will need to configure the SwaggerUI object by running an "unbundled" version of the app. You might think of this as creating a custom entrypoint to a docker container, say.

For example, the link you provide shows suggestions for running a customised version of SwaggerUI. Those customisations are written in JavaScript, so any old HTML page with the necessary dependencies in place which loads the script you write to configure SwaggerUI would answer the question of "where to put that code".

The details would depend on any frameworks you are or are not using.