How do we handle possessive older kids in communal play areas?

So specifically I’m talking about places like children’s area of the library, indoor free play, playgroup type situations.

Every time I take my 18m old to the library we run into kids who either in a group or alone have effectively “claimed” certain toys. These are usually older kids (5 or 6 year olds) When my kid walks up to play they are met with “these are mine” in which “these” refers to everything associated with that toy (I.e. play kitchen and all accessories, all the blocks at a table) or they have taken several things and have created an elaborate set up in which all related items are off limits.

Usually this ends with the older child wrestling the toy my child has picked up or is reaching for out of his hands or chasing him down to take it away from him. Sometimes they will physically block the area like a guard. Sometimes it’s not clear which toys are off limits and I’ll hand him a toy that then immediately gets snatched away.

I’ve gotten really good at redirecting and obviously kids don’t have to share if they don’t want to. Sometimes we’ll sit and “watch” the kids play. But what do you do when a kid has decided that no one else can play with something large that is designed to be shared with everyone? It’s really hard to get a young toddler to understand why sometimes they can play with something and sometimes they can’t.