Do two components using the same custom Hook share the same state?

Direct answer

No. A custom Hook shares reusable logic, but every component call receives its own Hook state unless that state is connected through a shared external source or context.

Calling the same custom Hook twice behaves like calling built-in Hooks twice: each call belongs to the component instance and has its own state slots.

Shared automatically: The Hook implementation and behavior
Not shared automatically: useState/useReducer state created by each Hook call
Can be shared deliberately: Context, an external store, URL state, server cache, or another shared data source