Find the calendar you like. You can choose between calendars that you can fully customize or predefined templates that you can start from. The calendar collection will be improved over time, so don't hesitate to come back from time to time!
As for the predefined templates, they are simply templates that already come with the material as you see it in the previews. They can serve as inspiration, whether it is by their image(s), the color theme used or the layout of the template itself.
Some models, beyond their apparent resemblance, have their own options. By browsing the list of predefined templates, you will have a quick look at the possibilities of each one. Note that for ease of use, calendars from the same family are displayed in a grouped fashion.