Does the plugin add content to each website page?

The answer to this questions (obviously) is yes as we have to output readable structured data so that search engines can read it.

However the data that is outputted is not visible to the end user. It’s only readable by search engines.

If you would view the sourcecode of one of your pages, you would find lines starting with something that looks like this:

<script data-snippet_id="snip-59a6a8000b284" type="application/ld+json">...</scrtip>

The data that can be found between these lines is the actual structured data that is formatted in JSON+LD format.