how to make fillable pdf using pages app

Pages lacks any form design capability other than creating a custom template with areas of placeholder text that one clicks on and enters replacement text - once.

If you want real inline document forms, then these are done with Microsoft Word, or Office clones like LibreOffice Writer, or Softmaker's Office 2024 products which have bonafide form design functionality built into the application. Or you can add form functionality in an exported PDF via Adobe Acrobat Pro, and many other true PDF Editors.