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