WordPress templates

You can install WordPress themes through the admin on your WordPress store.
Most authors will include the zip file for the theme which you will need to upload to themes section on your WordPress site.

For more help and support getting started with changing theme you can also read up on the follow link to learn more. Link

HTML templates

Most of the HTML templates listed will usually contain a src folder which includes all different views or that template.

If the author has included a demo link to their listing, there you should be able view all of the different views that you will get with the template.

CSS, JS and image assets are also commonly included inside the listing so you'll have all of the dependancies for the views.
You can always check with the author to see if there are any other assets that are required but most will include everything in the zip package.