Hi maschera,
Regarding the act of concatenating chosen items of a user, see the discussion in this post by Robert that may help:
https://community.glideapps.com/t/merge-data-of-array-column-into-one-cell/8549/5?u=thinhdinh
Regarding sending messages to WhatsApp, you can see this thread to generate a WhatsApp link with a predefined message.
Hope it helps!