I don't know why they are non-verbal but suspect it is mostly neurological pathway issues. There are many people here far more knowledgeable than I am, but here's what I do.
With my non-verbal nephew who is now about 30 years old, I came to realize that trying to talk is exhausting and stressful for him, that is rude and unfeeling to try to make him talk, and to ask him "yes" or "no" questions as much as possible.
Examples:
For lunch today, I can make us salads, fish tacos, tuna sandwiches, or ham and cheese sandwiches.
Do you want a salad?
Do you want a fish taco?
Do you want a tuna sandwich?
Do you want a ham and cheese sandwich?
OR
While you're staying with me today, we can take a long walk, or watch TV, or I'll read you a book, or you can work on a jigsaw puzzle, or you can look at Wikipedia on my laptop, or you can help me make cookies.
Do you want go for a walk?
Do you want to watch TV?
Do you want me to read you a book?
Do you want to do a jigaw puzzle?
Do you want to look at Wikipedia?
Do you want to help me make cookies?
He communicates enough that I usually can figure out what he really wants. He is getting better at volunteering words without being prompted. He ate lunch with me before the quarantine began and then said "I'm fed up!" I said "oh, no! Is something wrong?" He said "no, no, no! I am full!" I laughed (and he did, too) and I told him that there is a big difference between being "fed up" and "being full". He has a great sense of humor and I think he already knew the difference between those two phrases. He simply misspoke and he knew it almost immediately and corrected himself.
I sure miss not being able to spend time with him these days.