When you visit a website without using a VPN/Proxy/Tor, the website can see your public IP address. That public IP address is unique (with exceptions I’ll get to in a moment) to your home router. NAT means that each device connected to your router (Wi-Fi) has a local IP address, hidden to the website, but your routers IP is still unique to the website. That means that, even if you switch devices, if you visit a website using your home network the website knows that it is your Wi-Fi and not somebody else’s. That means that you can get tracked across websites just by correlating public IP addresses. Ads can see this IP address too. The public IP address by itself is enough to narrow down your location to the exact city, in most cases. So, when you visit a website, the website knows
The city you live in
Can correlate your public IP address (ad networks usually do this, not the website itself) to all the other websites you’ve ever visited
If your ISP uses dynamic IP addresses, that means your public IP address changes every month or so, so that #2 only has a history of about a month. CGNAT (Carrier-Grade NAT) means that multiple routers share the same public IP address, which removes #2 altogether. This still lets websites know the city you live in, but it reduces mass internet surveilling.
I may have gotten a few minute details a bit off, but that’s a basic shake down of how it works. TL;DR: Your IP can uniquely identify each of your devices if you don’t have NAT, your router if you do have NAT but not CGNAT, and the city you live in. Find an ISP that uses IPv6, dynamic IP addresses, and CGNAT, and use a elite proxy, free VPN, and Tor with a private DNS for maximum privacy.