I am building a plugin, but despite several search parameters, I cannot figure out how to replace a page's content with custom content generated by my plugin.

I have already written several shortcodes and widgets, but I'd like to replace the whole page.

The best example I have is CiviCRM. When the plugin gets installed, it generates a placeholder page which is replaced by custom content, I believe based on the query string.

I assume there's an add_action or similar add_ function I need to call for the plugin function to be called when the page loads.