It's in their interest to be vague but at the same time claim lots of source.
Some United States ISP sell anonymous browsing data. The data is still grouped by home connection. With SSL these days that only contains domain names but in the past it contained full URL. One was able to correlate if somebody searched for a product on shop A but then finished checkout on shop B. 15 years ago I dealt with such data, kind of scary. So you can correlate that people who visit one domain regularly also visit certain others.
DPS sounds like data processing, so intermediary that resells data or summaries. For example they might have data from browser toolbars, widgets on multiple websites or anybody else who sells user data.
Google Analytics: when you crawl pages you can extract the GA id and some companies use the same id on multiple domain. Thus you correlate they have the same owner. Similar with any other type of id or apikey one might use on the website, e.g. Google Maps API key.
Add some data on domain-to-IP address to see if a two websites are hosted on the same server.
> How can ISP's provide the data?
In the US it's part of the terms of service
https://www.netzero.net/start/landing.do?page=www/legal/your...
"we have collected the following categories of personal information from its consumers within the last twelve (12) months: "
- "Age (40 years or older), marital status (title), sex (including gender, gender identity, gender expression, pregnancy or childbirth and related medical conditions)."
- "Browsing history, search history, information on a consumer's interaction with a website, application, or advertisement."
- "Profile reflecting a person's preferences, characteristics, psychological trends, predispositions, behavior, attitudes, intelligence, abilities, and aptitudes."
"We share your personal information with the following categories of third parties:
Service providers.
Advertisers.
Affiliates.
Partners."
I'd argue all information an ISP has no business with.