When countries are being established by a political group, often assets such a manufacturing, agriculture, raw materials, etc. are considered and not the ethnic background of the residents. As I understand, the Dombas area of Ukraine is mostly Russian and that’s why they have been trying to separate from Ukraine and that is why the war from 2014 to present.
Yes, the Donbas is indeed mainly inhabited by Russian speakers. Evolution of language laws in Ukraine since incependence:
https://www.hrw.org/news/2022/01/19/new-language-requirement-raises-concerns-ukraine
This does not mention increasing pressure on Russian speakers, including social pressure to change their names to sound more Ukrainian, etc., which ramped up after 2014. Western journalists visiting cities such as Odessa noted such pressure had increased even more in the year or so preceding the Russian invasion, as well as prejudice against Russian speakers.
As for borders, Ukraine was never its very own country until 1991. The borders have shifted over the centuries (and decades). Various parts were inside various empires. The very name of the new country, Ukraina, means on the borderlands/at the frontier. It straddles ancient fault lines between religions and empires.
I posted some maps from WaPo awhile back that show how what is now Ukraine was ruled by various empires. I personally prefer old maps, but these show the outline of the present-day country of Ukraine in green, so you can more easily see which parts once belonged to whom at various stages:
https://freerepublic.com/focus/chat/4115436/posts?page=36#36
More here (but no maps, and other things discussed):
https://freerepublic.com/focus/chat/4115436/posts?page=18#18