Custom text/html in custom form module
What kind of entry (or #type) should I insert to get just an custom
  html/text block between fields or above them all?


Item element type provides a display-only form element with an optional title and description.

$form['help'] = [
  '#type' =&gt; 'item',
  '#title' =&gt; t('Block title'),
  '#markup' =&gt; t('Block content'),
];


You may also add arbitrary content to render arrays using the #markup render array key as follows.

$form['text']['#markup'] = t('Some text.');