in the doc it is mentioned that

You can only add BE nodes to shared-nothing clusters. Adding BE nodes to shared-data clusters is not recommended and may lead to unknown behaviors.

but most of the example configs in the helm chart deploys all 3 nodes, fe, be, cn. Can all both BE and CN exist in shared data or shared nothing? or is it just one or the other?

No, don't mix CN and BE on the same cluster.