Pat the shrimp completely dry with paper towels.
In a large bowl, whisk together the cornstarch, salt, white pepper, black pepper, sugar, and Sichuan peppercorn (if using). Reserve a pinch of the mix for finishing.
Toss the shrimp in the seasoned cornstarch until lightly and evenly coated; shake off excess.
Preheat the air fryer to 400°F (200°C) for 3 minutes.
Arrange the shrimp in a single layer in the basket (work in batches). Spray generously with oil until no dry cornstarch spots remain.
Air fry 6–8 minutes (8–10 for shell-on), flipping and re-spraying halfway, until golden, crisp, pink, and opaque. Internal temperature should reach 145°F (63°C).
Meanwhile, heat 1 tablespoon oil in a small skillet over medium-high heat. Fry the garlic and chili 60–90 seconds until the garlic is just golden. Turn off the heat and stir in the scallions.
Add the hot shrimp to the skillet and toss 30 seconds to coat with the aromatics. Finish with the reserved pinch of seasoning mix. Serve immediately.