Explain 'Symbol' types and their use in building private API surfaces.

Symbols create unique, immutable identifiers that can be used as object keys to prevent property collisions.

In TypeScript, the unique symbol type allows for 'opaque' identifiers. This is an architectural pattern used to create properties on objects that cannot be accessed or overwritten by accident because the 'key' itself is a unique reference. This is superior to using string keys like _privateProperty, which are still enumerable and accessible via normal means.