Communism as Marx envisaged is explicitly international in that it calls for the building of international democratic (i.e. worker-centric) political structures.
In the earlier years of the USSR, it openly sought international ideological supremacy, the cascading of the Soviet revolution, the collapse of capitalism, and the integration of further territories into the Soviet system.
As time went on, these hopes and efforts were moderated by the fact that the capitalist world itself increasingly used the state to organise and develop the economy, to redistribute wealth to workers, and to provide security against unemployment and sickness - all reducing the attractiveness or advantage of any communist transition - whereas the USSR itself rowed back from the most radical economic experiments which failed.
Stalin it seems soon recognised that the primary task was to preserve the Soviet system as a model covering part of the world, rather than assuming the USSR was in a position to conquer the entire world.
The USSR was a nation insofar as it was a power in the world amongst others, rather than a single state that governed the entire world. And once it became obvious that this would be the indefinite status, it optimised itself largely along the logic that applies to other nations.
The PRC meanwhile was nationalist from the outset. There was never an ambition for global conquest under a single Chinese state, and unlike the USSR the current territory covered by the PRC has significantly more historical tradition of being governed as a single state called "China".
The PRC have also had relatively sympathetic land neighbours from the outset (at least from a perspective of economic ideology and attitudes towards Western liberalism), a gargantuan population that rivals that of all Europe and North America, and came into being after WW2 and the development of nuclear weapons, meaning that concern about a successful capitalist military attack never predominated as it did for the USSR.
Finally, "ethno-nationalism" is broadly the nationalism as would have been understood by the term before WW2 - in other words, it's a Hitlerian ideology which categorises the world into races (or "ethnicities") and seeks to align (and maintain the alignment of) the scope of states and political territories with these races.
Ethno-nationalism since WW2 is broadly associated with the politics in colonial outposts like Northern Ireland or the former Rhodesia. It typically provokes war in its locality (as it did frequently when it reigned in Europe), and can be sustained nowadays only by transfers from a larger patron with different and more stable politics.