You can build a Next.js app and run it on a docker container or regular linux host almost anywhere. Vercel has some nice continuous deployment stuff built-in but I'm not sure how a Next.js app is locked into it at all.
Next.js and Vercel heavily push serverless deployment: 13 reworked the built-in API support to leverage Web Standards, which discarded interop with the a much larger server ecosystem in order to enable better edge support.
Yeah on the same note I developed a moderately complex app on Next but I hit a roadblock when I needed background job support, which is not natively supported (or at least at the time wasn't) on Vercel/other Next platforms and so it was never a priority for Next. Pushing serverless so hard also made deployments janky and production bugs weird when you tried to use things not supported by the underlying platform, AWS (don't remember the details now, but Node version was one of those).
