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'
    ],
]

and changing

'default' => 'my_new_theme',

Twig is used for templating