Typical web servers like Apache and Nginx support virtual hosts (aka server blocks in Nginx terminology)

Yes, traefik can do that as well - just point your domains to the VPS and set up yout traefik rules. Traefik will properly create certificates and route accordingly.

Yep, what you're looking for is a reverse proxy. All the incoming traffic to the server goes though it, typically on TCP port 80 or 443. Reverse proxy then directs the traffic to all the backend apps. The backend apps can be literally anywhere else or even on the same server on a different port.

Traefik, Caddy, Nginx Proxy Manager, Linuxserver SWAG are the popular choices these days. See which one fits you best.

Edit: Overlooked you mentioning you are already using Traefik. Subdomain, domain, makes no difference. Just point the DNS record to your VPS and have Traefik direct it towards the correct port on the same server.