Cosmology: Three-parted World
The idea of the three-part world is shared by Uralic cultures, including the mythology and religions of the Finns. The world is separated into three parts, in horizontal as well as vertical axis, simultaneosly. "Keskinen", "the middle world", our known world; "Alinen", "the underworld"; and "Ylinen", "the above world". These aspects of the world place themselves not only on the vertical axis, but horizontally: while the "underworld" exists literally under our feet, it is also situated in many mythologies in the far north, while the "ylinen" exists, in accordance with the established dichotomy, in the south.
As the world is separated in three, simultaneously there is the view of the world being made of two natures. "Tuonpuoleinen", literally "the other side", and "tämänpuoleinen", "this side". "Tämänpuoleinen" is what is our known world, while everything sacred and unknowable exists more or less "on the other side". "Keskinen" thus is of "tämänpuoleinen", while "Alinen" and "Ylinen" and all their inhabitants are of "tuonpuoleinen" in their nature.
People in general seemed to be more familiar with the Alinen. There were stories of shamans traveling there, the "geography" of the place was mapped out with it's river and gates, and it was where souls went after their death. People would leave offering to the inhabitants of the Alinen, to different "underground" spirits and to their ancestors under conifer trees (more about conifers here). Ylinen was in turn something so otherworldly it wasn't really approached. In these traditions the creator God was not something one approached much - it was a distant being, a deus otiosus - and the above world was its domain.
In my personal view: The role of the creator god in this Cosmology is fitting for the Abrahamic God/Jahve/Sebaot/The Lord etc. Unlike many gods in polytheistic religions, this God is completely otherworldly and in its nature distant from humans, an unknowable, omnipotent being. I think he fits as the "ruler" of Ylinen, which syncretic Christian belief might substitute with the word Heaven. The other realm of tuonpuoleinen, Alinen, would be the domain of spirits, other creations which are supernatural in their nature. I would see Alinen as existing, in a way, alongside and "under" our observalbe reality. This is where the souls of our ancestors and other "väki" would recide, "maanväki" (force/people of thre earth) most importantly. Keskinen then is simply everything that is NOT supernatural in nature. Our selves during our lifetime, and all the other inhabitants of this Earth.
Source: Pulkkinen, Risto: "Suomalainen kansanusko: samaaneista saunatonttuihin." Gaudeamus Helsinki University press, 2014