Often, if you try to go to a non-existent domain, it’ll still return an IP address that loads a “this site doesn’t exist” page hosted by the ISP, often full of sponsored links, similar to a domain parking page.
It’s trivial to do this. DNS requests are unencrypted and can easily be modified by an ISP, even if you use a custom DNS server like Google’s 8.8.8.8 or Cloudflare’s 1.1.1.1. You need DNS over HTTPS or a similar technology to prevent this happening.