On the Marketing Calendar, you can see the 6 entity items at a glance.
(clicking the bullet point links will take you to the respective article on the Marketing Calendar Entity)
You can select and un-select the Calendar's Entities to show the type of view you are looking for (as expressed on our help article here too). The Entities shown within the Calendar cannot be customized, so you are not able to add or remove any Entities or Entity fields within this feature.
Within the Marketing Calendar, if you want to learn more about an item without leaving the calendar you can.
Click on the item you're interested in and you'll see a pop-up appear above/next to the item. It will tell you the name, Campaign (if it has one), and few other details depending on the entity.
If you want to visit the record related to the calendar item, simply select the View Full Record link in the pop-up and your window/tab will redirect to the related record.