Toss chicken pieces with vegetable oil and salt in a large bowl and leave to marinate for about 10 minutes.
Add cornstarch to the marinated chicken and toss until the chicken is unevenly coated, with some cornstarch still dry.
Arrange the chicken in a single layer inside the air fryer basket, avoiding overlap. Spray oil over the chicken to cover the dry cornstarch.
Air fry the chicken at 450°F (230°C) for 8 minutes until the pieces look pale yellow.
Flip the chicken or shake the basket, then continue air frying for another 5 to 6 minutes until golden brown.
Transfer the cooked chicken to a large plate and let it cool slightly.
Whisk together ketchup, Shaoxing wine or dry sherry, rice vinegar, soy sauce, brown sugar, water, and cornstarch in a small bowl.
Heat peanut or vegetable oil in a skillet over medium-high heat until shimmering.
Add minced garlic and ginger to the hot oil and stir for a few seconds until fragrant.
Add chopped bell pepper and white onion, cooking and stirring briefly for 30 seconds.
Give the sauce mixture a quick stir to dissolve cornstarch fully, then pour it into the skillet.
Cook the sauce, stirring continuously until it thickens.
Toss the cooked chicken into the sauce and stir everything together until all pieces are evenly coated.
Serve the coated chicken and vegetables immediately while hot.