Creating a simple custom template in Drupal 8 module

Custom template files in Drupal 8 uses twig files. Apart from that defining a custom template is similar to Drupal 7. Custom templates are called and values are passed to custom template in the Controller classes in Drupal 8 module.

Define the custom template in the modulename.module file using hook_theme as in Drupal 7

function lncustomtemplates_theme($existing, $type, $theme, $path){
    return array(
        'my_template' => array(
            'variables' => array(
                'test_var' => NULL,
            )
        )
    );
}

Call the custom template and pass the variables to custom template in the Controller class.

namespace Drupal\lncustomtemplates\Controller;

use Drupal\Core\Controller\ControllerBase;

class CustomTemplatesController extends ControllerBase{
    
    public function mytemplate(){
        return array(
            '#theme' => 'my_template',
            '#test_var' => $this->t("This is a test value from custom template variable"),
        );
    }
}

Create a html.twig template file with the same name as custom template in the src/templates folder. Display the passed variables using {{ var }}

<h3>Custom Template File </h3>

<strong> Test Var: </strong> {{ test_var }}

More information on custom templates can be found in Theme API in Drupal