Plato’s realm of forms is about pure, essential ideas that are like blueprints for everything actualized in any universe. Ideas can fit in a thought of God. Plotinus named God ‘ the One”, and elaborated the realm of forms in his book named The Enneads
Ideas of spirit exist eternally for The One. Yet like an entire universe that is designed and static in a zero-dimension singularity, no space-time is required to contain it. Space-time is part of the kernel of the Universe that unfolds dimensionally- though one wonders about the time in which time is encapsulated at a singularity.
Every quantum form is pre-determined from alpha to omega in any possible universe among the multiverse of possibilities God deems possible. An infinite number of thoughts that comprise forms are known unto the omniscient One and have always existed even as the sentient beings that occur within them wonder about space-time and the quanta and qualia of a Multiverse and of God likening it unto a simulation.