I started writing out a reply to this question focusing on the taco (since I don't know anything about poutine), but I realized that this is actually a difficult question that covers hundreds of years of food history, immigration, food systems, and cultural exchange across three cultures, which I could never hope to do justice to in a short response. But you might be interested in the book Planet Taco: A Global History of Mexican Food by Jeffrey Pilcher. Aside from having the best title, it looks at the globalization of Mexican food, with a focus on Mexico and the United States, and later the rest of the world.

Pilcher describes a complex three way process of the popularization of Mexican food in the US. First was from the bottom up, led by immigrants (and also people who were living in the northern part of Mexico, which then became a part of the US after the US/Mexican War) who brought new cuisines into the Southwest US (and later other parts of the country) in many immigration waves for almost two centuries. Secondly, from the mid 20th century up to today, Mexican food has spread from the top down as corporations and small businesses alike worked to popularize Mexican food, normalize immigrant cuisine, and shape American diets, especially through fast food and mass production. Thirdly, Mexican cuisine was embraced by American eaters, who co-opted and mixed it with American ingredients. This formed a "safe" way to experience exotic culture without breaking the boundaries of segregation. At the same time, eating out became (and continues to become) more common than it was in the early 20th century. Mexican food benefited from this trend, going viral in a sense. The story of tacos is much more complicated than this summary though, and I think Pilcher does a good job hitting a lot of the major themes with the book.