Let's say for the sake of discussion that Jacob had twelve sons.
Almighty God told Jacob a nation and a company of nations would proceed from him.
"I am God Almighty. Be fruitful and multiply; a nation and a company of nations shall proceed from you, and kings shall come from your body." (Genesis 35:11)
This means a nation and a company of nations had to proceed through one of those twelve sons.
Let's see if we can narrow it down.
"Now the sons of Reuben the firstborn of Israel — he was indeed the firstborn, but because he defiled his father’s bed his birthright was given to the sons of Joseph, the son of Israel, so that the genealogy is not listed according to the birthright." (1 Chronicles 5:1)
The birthright should have gone to Reuben since he was the eldest son, but because of his incest he disqualified himself.
So the birthright belonged to Joseph.
"Now when Joseph saw that his father laid his right hand on the head of Ephraim, it displeased him so he took hold of his father’s hand to remove it from Ephraim’s head to Manasseh’s head. And Joseph said to his father, 'Not so, my father, for this one is the firstborn, put your right hand on his head.'
But his father refused and said, 'I know, my son, I know. He also shall become a people and he also shall be great, but truly his younger brother shall be greater than he and his descendants shall become a multitude of nations.'"
Manasseh was to become a great nation and Ephraim a multitude of nations.
Based on these scriptures, who do you say is the great nation and the multitude of nations.
Looking at the world today it seems simple to me.
You went from attempting to sort all that out through the Scripture, to the newbie error (no matter how much time such may or may not have in Scripture, Heb. 5:12-14) of "well, what it means to me is" and or "perhaps it means..." - which you did by your clearly obvious turning from the Scripture to your stated "Looking at the world today it seems simple to me."
I would suggest you go back and reread throughout the WHOLE of Genesis as to who was promised what.
In this, the guiding principle is that the latter is understood in the earlier.
Rom. 5:8
Acts 17:11,12