Read only fields like created by and last modified by are visible as read-only on edit page in lightning but not in classic and mobile app (edit page is not overridden and I am logging in with same user in mobile app and desktop) . Is there any way I can make them visible in classic and mobile app as well.
System fields are not displayed in edit page layout. You can only see them in detail pages. You can have a custom field which will be updated by last modified date whenever a record is update by using workflow rule and can use the same to show user the date on edit page. Make the readonly from page layout to avoid changing the values.