Can you point custom user domains to directories instead of subdomains?

Just wondering if it’s possible before investing the time learning how to implement it. Building website in Next.is. Users are given their own page like myapp.com/user123. Is it possible for users to point their own domain to that like user123.com. I know it’s typically possible pointing to sub domains but I’d rather point to a directory path like that. Also if I allow users to change their username after the fact how much complication would that add or would they just need to update dns records?