All HTML templates are found in the resources/theme/default folder. You may add your own theme to a new folder in the resources/theme folder. If you chose to do so, you must also modify config/themes.php and add your theme to the themes array
'themes' => [
'example_theme' => [
'extends' => 'default',
'views-path' => 'resources/themes/example_theme',
'asset-path' => 'public/example_theme'
'default' => 'my_new_theme',
Twig is used for templating.