I agree there should be a manual. My phone (Samsung) has an almost useless pdf manual available.
But I also think that most people do not spend anywhere near the time necessary to learn how their phone works or how to use it. They wouldn't read the manual even if it was available. They don't watch the intro videos that play when you first start your phone out of the box. They expect to just get it out of the box and it to magically be completely usable for them.
There's lots of ways they could learn those things you mentioned. Libraries and community centres often have courses for beginners. There's youtube videos. There's books. There's endless websites. Nothing makes most techies special except the motivation to google the things they don't know. Most people don't even try, and just muddle along with what little they already (don't) know.
How can you learn to press and hold , or press harder, or shake the phone, or whatever else when there's no manual, you don't even know there's some other way of interacting, there's no getting started video, etc. I firmly believe that if companies actually wrote a manual for their software, they would ocassionaly make things more obvious, so it would be easier to write the manual.
Yes, a lot of people wouldn't read the manual, or wouldn't understand it, but not providing a manual means those few that would read it can't.