How do I check radio buttons in Twig template?
That's a task for #states not for Twig. I think this also could be ajaxified a little bit more elegantly but using #states on some items does the job as well.

$form['payment'] = [
  '#type' =&gt; 'radios',
  '#title' =&gt; $this-&gt;t('Payment Mode'),
  '#options' =&gt; [
    'chk' =&gt; $this-&gt;t('by check'),
    'tfr' =&gt; $this-&gt;t('by bank transfer'),
    'crd' =&gt; $this-&gt;t('by card'),
  ],
  '#required' =&gt; TRUE,
];

$form['chk'] = [
  '#type' =&gt; 'item',
  '#markup' =&gt; $this-&gt;t("You chose 'Check'"),
  '#states' =&gt; [
    'visible' =&gt; [
      ':input[name="payment"]' =&gt; ['value' =&gt; 'chk'],
    ],
  ],
];

$form['tfr'] = [
  '#type' =&gt; 'item',
  '#markup' =&gt; $this-&gt;t("You chose 'Bank Transfer'"),
  '#states' =&gt; [
    'visible' =&gt; [
      ':input[name="payment"]' =&gt; ['value' =&gt; 'tfr'],
    ],
  ],
];

$form['crd'] = [
  '#type' =&gt; 'item',
  '#markup' =&gt; $this-&gt;t("You chose 'Card'"),
  '#states' =&gt; [
    'visible' =&gt; [
      ':input[name="payment"]' =&gt; ['value' =&gt; 'crd'],
    ],
  ],
];