An ISP proxy differs from a basic proxy server by using an IP address that is hosted in a datacenter but announced on the network of real carriers and ISPs.
Most basic proxy servers are hosted in datacenters and use IP addresses that are announced on the networks/ASNs of datacenters.
There are several online services that allow websites to check IP addresses and whether they are assigned to datacenters or residential ISPs like Comcast, Verizon, Spectrum, Charter, BT, Virgin Media, and many more.
Websites use these services to assess whether the IP addresses connecting to their site are real customers and some websites block access or restrict content to connections that are detected to be from datacenter networks.
An ISP proxy differs from a basic proxy serverby using an IP address that is hosted in a datacenter but announced on the network of real carriers and ISPs.
This means connections from ISP proxies look like real customers to websites and make it much harder for them to block access or restrict content, making the experience of using an ISP proxy much better than a standard datacenter proxy on some websites.
ISP proxies are sometimes also called static residential proxies.