You can put variable length data into a struct:

https://en.wikipedia.org/wiki/Flexible_array_member

You definitely can but it's not as obvious, make a separate message type for list elements and append them on the wire. If you only have one list at the tail, you can use a flexible array[] at the end but it's finicky to deal with if you need more than one.

You can build large hierarchical structures of messages with lists contained therein. It's pretty much how .mov/.mp4/many, many media container formats work. The technique dates back to the Amiga days.