Ukko is an MNO, not an MVNO. They hold a national license for the 450 MHz band which they use to provide data services in all of Finland.
Here is their coverage map: http://www.ukkoverkot.fi/kuuluvuus/
> If they indeed operate on the LTE 31 band this also means they are a speciality carrier which also means they have to provide you with their own devices since I'm not aware of any mass market consumer device which supports B31
Correct.
> The cellular spectrum especially in regards to 4G LTE channels is pretty damn large, and most countries started with more than 3-4 cellular providers they just had decades of consolidation since then.
Not all LTE bands are available in all countries.