This is a good use case for the WP REST API. Site A’s content is available through the API, so if you know the ID of the page you want to use, you can make a request for the content that way.
For example, if your About page has an ID of 2, then hitting https://sitea.com/wp-json/wp/v2/pages/2 would return a JSON object with the data for the page. If your page on Site B makes a request to Site A, gets the response, and displays it (without pulling it into Site B’s database), then you’ll always be getting the latest version of the content. There are some performance issues to look for, since you’re making a remote request on every load, but it’s possible to cache the response for a period of time and use the cached version to speed up the local page load.
I recommend reading through the REST API handbook for more details on how the API works and how to integrate it into a theme or plugin.