Before we even realise it, our brains are constantly scanning the information on the screen in front of us. It's recommended practice to place the most crucial action last on small windows.
So, bottom and right, is the best position as human eyes scan content from left to right following the Z pattern! Right position ensures users will follow a simpler path in order to find the primary button as it is shown in the picture.
On the other hand, on larger browser windows and especially if users have to scroll down, the primary button should be placed directly beneath the final paragraph of text on the left!