Facebook Pixel work:
The principle behind Facebook pixel is similar. The pixel is a transparent, 1x1, unique image file that can be embedded on pages outside of Facebook (unique = 1 per advertiser account). That image file, however, sits on Facebook servers. So, each time it is loaded, it increments counters on Facebook's side.
But there's more!
Each time a user accesses Facebook, using a specific browser, machine, and IP address, this generates a signature that Facebook matches to that user. If the same user uses several devices and several browsers, and don't we all, all of these combinations can be associated to one, single user.
And each time the pixel file is being seen by a user... Facebook servers can see which browser is used, which machine and which IP address. In other words, they are able to reconstruct that signature - they know which Facebook user has seen the pixel. As long as a user accesses the page that has a pixel in a way that they used once to log on Facebook, they can be identified and matched to a Facebook ID.
But that's not all!
When the pixel is seen, Facebook server is also aware on what page it is. Informations on the page can determine what action the user was performing.
So Facebook can know things like: who are the users who checked a given product in the last 2 weeks but didn't purchase it? And advertisers can show an ad to that specific group. And that ad can be cross-platform: if you've checked a pair of shoes on your phone while in the bathroom (... it's ok we all do that) but didn't click play because you couldn't see well enough, you can get an ad for that same pair of shoes next time you connect to Facebook on a computer.