How do I format the display and submission format of a datetime field in a custom composite?
For defining the format of the DateTime field you can use CUSTOM COMPOSITE SETTINGS.

Example: To set the format to datepicker like a normal date field, set datepicker: true
inside the custom text field for the respective composite DateTime element. All other properties can be added here.

You can checkout the JSON representation of the respective property from the source option of the webform. Copy-pasting the properties inside the custom composite setting is all needed.

To set the submission format:

Go to Advance option of the composite element > Under submission display fieldset select the way you want to display the items.