To assign default permissions to pages in Typo3, the following TSConfig snippet is helpful:

TCEMAIN{
  permissions.userid = 1 # Backend user with ID 1
  permissions.groupid = 1 # Backend user group with ID 1
  
  permissions.user = show, editcontent, edit, new, delete
  permissions.group = show, editcontent, edit, new, delete
  # no permissions for 'other'
}

This code should be placed at Edit Page > Resources > TypoScript Configuration.

After saving the configuration, any new page below the edited page receives the configured default permissions.