I know Vercel uses AWS Lambda's behind the scenes to process web requests at least. I'd assume caching is also handled through Cloudfront by default. The place I currently work uses Fastly for caching and Vercel for hosting and it's definitely caused some issues(and much finger pointing on both of their sides) when one of those services makes a breaking change.
Perhaps they're using CloudFront and providing an end user CDN on top of it. Both are partners with AWS. Both were multicloud, and it seems, aren't anymore.
Cloudflare is already on the way to building out all of the features Vercel has which is exciting. Eventually their Pages product (static hosting) will integrate Workers [1] for serverside APIs.