Cook the bacon in a dry skillet over medium heat until deeply crispy. Drain on paper towels and chop roughly. Set aside.
Cook the fries according to package instructions, or air fry at 400°F for 14 to 16 minutes, shaking halfway through. If making from scratch, bake at 425°F for 30 to 35 minutes, flipping once.
Make the cheese sauce: melt butter in a small saucepan over medium heat. Whisk in flour and cook for 1 minute until pale golden.
Add warm milk slowly while whisking continuously. Cook over medium heat, whisking, until the sauce thickens and coats a spoon, about 4 to 5 minutes.
Remove from heat and stir in shredded cheddar, cayenne, and garlic powder until completely smooth. Season with salt and pepper. Keep warm on lowest heat.
Layer the fries: spread half the hot fries into a wide shallow serving dish. Pour one-third of the cheese sauce over, then scatter half the bacon and half the green onions on top.
Add the second layer of fries and repeat with remaining cheese sauce and bacon.
Finish with jalapeño slices, sour cream, and the rest of the green onions. Serve immediately.