If you have a bag of sweets, you offer them round. If you have your favourite teddy, you don't have to share that. If someone comes to your home, you share your toys. If anything is particularly precious, put it away in a cupboard before they arrive. If there's only one thing and both of you want it, you take turns. If you really want something somebody is playing with already, you wait a while and see if it becomes free.
You don't demand, snatch, make a fuss if you can't have something. It's a bit like going to a gym. You want to use the rower but somebody else is on it. So you go and do the treadmill for a bit and see if it becomes free. If somebody's hogging it, it's ok to ask, may I have a go. They might say no, in which case you have to accept that.
Which is entirely different to a friend's view. Her view was if her ds asks for it nicely, he can have it. No matter who it belonged to, what the circumstances were, how long the other dc had been playing with it. That to me is not sharing. He came a cropper when he started school and other dc wouldn't get off the activity tables to give him a place at his demand.
I think if you take something to the park or the beach, then no you don't have to share that with random strangers. Neither is it ok to go and take somebody's ball, scooter, bike (we've had that) or rubber dingy and start playing with it. But if you've gone with a friend, yes you do have to share these things.
It must be very complicated for small dc I think.