Where are JS primitives and reference types actually stored? Stack vs. Heap vs. Execution Context (AO/GO)

I am confused about the storage of JavaScript values. I have seen two different explanations:

Explanation 1:

Primitive types are stored on the stack.

Reference types are stored on the heap.

Explanation 2:

If a variable is global, it is in the Global Object (GO) -> stored in the heap.

If it is in a function, it is in the Activation Object (AO) -> also stored in the heap.

Which one is correct? Is this a difference between a conceptual model and the actual engine implementation?