The Final Multiverse is a verse that contains The Final Universe, and here is the place where the verses finally return to physicality, due to its properties of having something called a physical vacuum around it. It contains many Final Universes.
Physical vacuums are a sort of barrier that allow for certain laws or ideas to be transferred from one type of concept (like abstract) to another (like physical). They seem to only come in specific sizes, all of which are about the size of a Final Universe.
When transferring from one type of concept to another, the way that size, dimensionality, etc. are measured may change, allowing for The Final Multiverse to actually be smaller than the Final Universe (although they cannot be completely compared due to it being a different type of concept).
The structure of The Final Multiverse is actually extremely similar to that of a normal Multiverse, where if you replace the Final Universes and physical vacuums with normal Universes and shrink the verse down, they will look nearly identical from the inside. One difference between the Multiverse and The Final Multiverse is that the barrier of The Final Multiverse is much harder to pass, being almost as hard to break as a Lockverse.