Hello again!
It depends on which view or views you would like to customize. In the link you provided, you have the “month” view set as the default. To customize that template, you would copy the template from wp-content/plugins/the-events-calendar/src/views/month/content.php into your-child-theme/tribe-events/month/content.php.
Then just add any custom markup in the template where you want it to display.
Let me know if this answers your question, or if you have any others.
Thanks!
Sky