Place shrimp in a large shallow bowl and season well with salt and pepper. Add 3 tablespoons sweet chili sauce, soy sauce, and garlic to bowl and toss until shrimp is evenly coated. Allow to marinate for 20 minutes, tossing halfway through marinating.
Toss zucchini with vegetable oil and season with salt and pepper to taste. Preheat an outdoor grill to 400F OR lightly grease and heat a cast iron grill pan over medium-high heat on stovetop if cooking indoors.
While shrimp is marinating, grill zucchini on preheated grill on a sheet of tin foil or a grill pan or on stovetop 5-7 minutes, brushing with sweet chili sauce occasionally during cooking and turning frequently until zucchini is lightly charred and tender. Remove from grill, cover to keep warm, and set aside.
Remove shrimp from marinade and discard marinade. Grill shrimp on a sheet of tin foil or in a grill pan or work in batches to sear in preheated pan on stovetop. Brush shrimp with a little additional sweet chili sauce during grilling, turning once until shrimp is lightly charred on both sides, about 3-5 minutes of cooking.
Arrange shrimp and zucchini on a serving plate or in a bowl and drizzle with a little additional sweet chili sauce for serving. Top with green onions, red pepper flakes, and sesame seeds and serve warm. Enjoy!