The site operator asks Vodafone "what is the unique account ID for the machine accessing my site right now from 17.56.2.43:3452?", and Vodafone gives them an account ID. They can then use that account ID to correlate to previous interactions you had with their site, even if they were coming from different IPs.

Https and DoH don't protect you in any way from the site operator wanting to serve you ads, and Vodafone will always know what IP:port they assigned you personally (well, your phone).

Tor, VPN and proxy services can protect from this, since they decouple your original request from what the server receives. Of course, the latter two can also sell your information instead of Vodafone.