Use of Twig template for webform email
Alright, after a lot of searching and testing things out i've found the following things to work:


For the checkbox it won't work with the data.element_name syntax (or at least in any variation that tried, and that's a couple dozen).  What will work is this: {{ webform_token('[webform_submission:values:ticketselectie]', webform_submission) }}.  In this example "ticketselectie" is the name of the checkbox element with 4 checkboxes in it (options).  What you get from this is a string containing the option text for each checked checkbox, seperated by comma.


You can use the twig split filter to extract an array out of checked checkboxes.
Turns out you need to use data.custom_composite_element.0.element_key as the syntax.  given that a custom composite element only ever has one row, it's always the 0 delta you need, and then the name of the key behind that.  So given a custom composite with the name "abonnementen", and in there two number fields with the names "minderjarigen" and "anderen", that leaves with the following syntax: data.abonnementen.0.anderen or data.abonnementen.0.minderjarigen



That's it really.  Took way too long to find that, and it's kind of weird to me that the "data." syntax didn't work for the checkbox element.  So if anyone knows a better way, please do reply.