Array not displaying in twig template
Several things are wrong here.

Your hook_theme must return:

return [
  'return_products' =&gt; [
    'variables' =&gt; [
      'products' =&gt; NULL,
    ]
  ]
];


Now you have a template called return_products with a variable called products.

In your controller you must then use '#theme' =&gt; 'return_products', instead of '#theme' =&gt; 'return-products',.

Then create a template in mymodule/templates/return-products.html.twig.

Notice how your theme name must be with _(underscores) when referred to in PHP, but the file name must be with -(dashes).