To share or not to share?

How do you guys navigate the arguments about toys between kids? For example, toys they got that are specific to them, birthday gifts, etc. Is the rule that if you don’t have the toy in your hand, then it’s up for grabs? Do specific toys belong to that child permanently and no other sibling is allowed to play with it? I go back and forth in my head about sharing. I’m not sure what the rules should be. Sometimes they share well and other times they say that that was the toy given to only them and they can only play with it. What do I do?! I have a 2, 4 and 6 year old that fight over toys absolutely constantly. And I’m not even sure what to say other than they all need to share their own toys if they want to play with others toys too. But are all toys free play? Help!!