Hey @Kathy,
Thanks for reaching out. Most of your questions can fortunately be answered in our comprehensive Themer’s Guide, which can be found here → https://theeventscalendar.com/knowledgebase/themers-guide/
Check that out to learn about making templates that don’t get overridden on updates and such.
To find event details or any other file, the best thing to do here is simply to explore the plugin files directly to find the file you need. There are many files but you only need to look in the /src/views sub-directory.
The Events Calendar itself has a file used for generating event details that exists at src/views/modules/meta/details.php in the plugin files, for example—something like this file might be perfect for your needs.
Best of luck with your customizing!
George