Probably you understood
@YarS in the wrong way. Of course Ukrainian is a separate language However it is very close to Russian in all respects.
I work with a man whose wife is half Russian, half Swedish. She speaks Swedish as a native Swedish speaker but she hardly can understand dialect of Swedish that is in use in Malmo.
There is a lot of similar examples when native speaker is unable to understand speakers that use some dialects.
By contrast, it is not a big problem for a Russian speaker to understand Ukrainian with exception of some Western dialects that many Ukrainians themselves don't understand.
What is dominant language in the UK? English of course, including Scotland, Wales,NI. Even in Irish republic Engish is dominant for very simple reasons. Standard French is dominant in France, Hochdeutsch is dominant in Germany and so on.
It was de facto situation and still most of big cities are predominantly Russian speaking even now and Ukrainian is in use mostly in country side. So even now Ukrainian is mostly language of farmers.
Relatives of my wife live in the big city of Dnepr and in nearby big village. Villagers use to speak Ukrainian and those who live in the city speak Russian. Then they meet they have no problems with language.