Cook the chickpea rotini in boiling salted water according to package instructions until al dente, typically 7 to 9 minutes.
Drain the pasta and rinse immediately with cold water to remove excess starch and stop the cooking process.
In a large mixing bowl, whisk together the Greek yogurt, ranch seasoning, and milk until the dressing is smooth and uniform.
Add the cooled pasta, diced chicken, cherry tomatoes, cucumber, and red onion to the bowl with the dressing.
Toss the ingredients thoroughly until the pasta and vegetables are evenly coated in the ranch dressing.
Gently fold in the shredded cheddar cheese and chopped baby spinach.
Cover and refrigerate for a minimum of 30 minutes to allow the flavors to integrate before serving.