1. Ad tech:
As I browse, my interests are accumulated into basic interest groups, demographic data, and maybe recently visited websites that use tracking.
The result: when I visit a news website, I might see an ad for tennis racquets (because of interest), or a local bank (because of location), or DigitalOcean hosting, because I recently visited that site.
2. Facebook's platform, used by Cambridge Analytica:
I install a Facebook psychological quiz app (supposedly from the prestigious Cambridge university), and give it permission to access my data.
Through that, Republican campaigns also access my grandmother's account, with all of her photos, likes, and comments. They use that data to learn her deepest fears.
They hire designers and writers to create fake news websites, and get their propaganda stories to appear in her Facebook news feed, where they appear to be legitimate right next to the news about the local sports team.
The result: when I visit for Christmas, I have to try to calm her down, that Hillary Clinton isn't actually running a late term abortion clinic out of a pizza parlor. And she won't believe me.