How GAMG Preconditioner in OpenFOAM really works?

Question

I use OpenFOAM for CFD simulations. A very popular preconditioner is GAMG which needs a low number of iterations per a time step in SIMPLE or PISO solvers that are used to simulate the fluid flow.

I would like to understand how GAMG prepares the internal structures.

Does it create the internal data structures used in GAMG only once in a single iteration on the basis of the geometry and then the entries in these data structures are only updated?

OR

Does it create the interal data structures multiple times in a single iteration? I read the documentation here but it does not have the answer.

Thanks for your answer.