How do I create a simple checkbox in a content entity? I just want a boolean checkbox with a label beside it. How do I do it?
Use type boolean_checkbox and display_label:

$fields[$open] = BaseFieldDefinition::create('boolean')
    -&gt;setLabel(t("My Checkbox Field"))
    -&gt;setDisplayOptions('form', array(
      'type' =&gt; 'boolean_checkbox',
      'settings' =&gt; array(
        'display_label' =&gt; TRUE,
      ),
      'weight' =&gt; '0',
    ))
    -&gt;setDisplayConfigurable('form', TRUE);