The Internal Page Cache is a core module in Drupal responsible for caching pages requested by anonymous users.
When a page is cached and an anonymous user makes a new request, Drupal does not need to perform any rendering or page-building processes. It simply retrieves the rendered page from the cache and sends it to the client.
The reason it only applies to anonymous users and not authenticated users is that the page returned to the client must have exactly the same content for all users.
In the case of authenticated users, although part of the content may be the same for everyone, there are always elements that can vary, such as the user block displaying the user's name or other user-specific information.
For these cases, there is the Dynamic Page Cache module, which handles caching for both anonymous and authenticated users.
Functionality
Cache Bin
For storing and managing cached pages, the Internal Page Cache defines its own cache bin called “Page,” meaning that cached objects are stored independently of other existing cache systems in Drupal.
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