“From one man he made every nation of men, that they should inhabit the whole earth”: Paul is connecting the unity of humankind with the universal sovereignty of God. God is one, humankind is one. There is one God for all of humankind.

“and he determined the times set for them”: History operates with both God's providence and human free-agency. There is a continual interplay between the various elements that result in what we call human history. In addition, God has determined the times and the boundaries where humans should seek Him and find Him.

“and the exact places where they should live”: Paul’s emphasis about political boundaries, nations, and empires is that there are no local deities, but rather one God (YHWH) who is sovereign over all of them.

Does God decide countries' borders or where/when we are born? Or both?

Acts 17:26 NLT From one man he created all the nations throughout the whole earth. He decided beforehand when they should rise and fall, and he determined their boundaries.

And he made from one man every nation of mankind to live on all the face of the earth, having determined allotted periods and the boundaries of their dwelling place, Acts 17:26 ESV

and He made from one man every nation of mankind to live on all the face of the earth, having determined their appointed times and the boundaries of their habitation, Acts 17:26 NASB2020

From one man he made all the nations, that they should inhabit the whole earth; and he marked out their appointed times in history and the boundaries of their lands. Acts 17:26 NIV