There’s a chain of reporting issue. Generally, the site you’re on (outside the social media shitholes) will keep track of some identifier of who you are, and which sites you visited. Then they will share your identifier with an advertising partner to deliver you advertisements. The advertising partner will take your data and pass it off to a data management platform (Hi Oracle!) who will then attempt to link you on that site to literally everything you have ever done. They have deals with Credit card companies, TV vendors, car manufacturers, cell service providers, public databases. That’s where the sketchiness happens. The worst part of it all is that realistically, the advertisers don’t care about all that data. They almost always want some very basic demographic data that fits in the old Nielsen family demographic data: It doesn’t make any sense to advertise a Lexus or investment advice to someone making minimum wage. Politicians want to know who likely voters are. Macy’s wants to advertise at people who shop at malls. They also want “Lives within 50 miles of my business”
The biggest worry is that the data platforms collect a lot of exact data that is not used except for super suspect Cambridge Analytica level targeted political advertising, and to add to it: They are reallllllly crappy at their jobs. I work in AdTech, so I can poke around at what they think I am and I’ve had things from “Salad Dressing lover” to being both unemployed and making $1 million/year in the same profile.