On the browser the Notes fields are rendered as objects associated with the form and are accessible via JavaScript. E.g. document.forms[0].NotesField.value = "hello" would set a notes text field named "NotesField" to the value "hello".
RichText, checkboxes and radio buttons are handled as array objects, so they must be indexed (e.g. NotesField[0]). You set different properties based on the field type ( .value=, .text=, .checked=true, etc.).