Erratically, doughnuts are often thought to be “all American” food.
As with most stuff culturally appropriated by those colonist-decendants, this is a claim that can easily be debunked by history books.

Doughnuts can be traced way back to Roman times BC where leaved dough fried in ample oil were heavily documented to be a popular snack.
Frying or baking dough in rings was also a common practice for at least a millennium in the middle-east.
The current recipe for what we now know as doughnuts came from the Netherlands, with Dutch settlers who brought their traditional “olykoeks” (oily cookies, currently more well known as “oliebollen”, which means “oil balls”) to the Americas approximately in the 19th century.
Fried sweet leavened dough have been known in many forms in many countries.
It’s really that popular as a snack for history knows how long.
From the French beignets, to the Italian fritella, the krofne from the Balkan regions, the Nigerian puff-puff, the sufganiyah as traditional Hanukkah food, the Portugese malassada, the Danish Æbleskiver, etc, etc…

The word “doughnut” itself is a playful combination of “dough” and “nut” which refers to the original use of nut-sized pieces of dough that were fried until golden.
But one day someone decided that a ring shape would fry a doughnut more evenly.
And that is how the “nut” in “doughnut” became a nutty relic that reminds us of the doughnuts’ “un-holy” origins.