Unfortunately, there’s a strong possibility that you don’t have that feature at all. The 2021 Cadillac XT5 does have ambient lighting—but only if you get the Premium Luxury trim level. If you just got the standard Luxury trim, you won’t have ambient lighting.
In addition to ambient lighting, having the basic Luxury trim means you’ll also be missing out on full leather upholstery, heated steering wheel, additional seat settings, and a wireless charging pad. Since you just got it, you might try seeing if the dealership will let your upgrade to a higher trim level.