Which field should I edit to make the checkbox marked by default?

It most likely must is 'default' => TRUE.

'request_vendor_access' => [
  'label'    => __('Request access to become an Artist', 'ignitewoo_vendor_stores'),
  'type'     => 'checkbox',
  'required' => FALSE,
  'class'    => ['form-row-full vendor_stores_request_access'],
  'clear'    => TRUE,
  'default'  => TRUE,
]

If not, also try 'default_value' => TRUE, or try it without 'clear'.

Depending on what you want to achieve you might also want to get the default value dynamically. So that it restores the user’s choice. Maybe get_option($option, $default) is the right choice here (if that’s an Options API form).

'request_vendor_access' => [
  'label'    => __('Request access to become an Artist', 'ignitewoo_vendor_stores'),
  'type'     => 'checkbox',
  'required' => FALSE,
  'class'    => ['form-row-full vendor_stores_request_access'],
  'clear'    => TRUE,
  'default'  => get_option('request_vendor_access', TRUE),
]

or

'request_vendor_access' => [
  'label'    => __('Request access to become an Artist', 'ignitewoo_vendor_stores'),
  'type'     => 'checkbox',
  'required' => FALSE,
  'class'    => ['form-row-full vendor_stores_request_access'],
  'clear'    => TRUE,
  'default'  => isset(get_option('my_option')['request_vendor_access'] ? get_option('my_option')['request_vendor_access'] : TRUE,
]