Consider the following specific scenario: -
You have installed Facebook Pixel javascript code on your website.
You have created a "website custom audience" in the Facebook Ads Manager so that you can show your ads on Facebook only to those people who have visited your website in the past.
Your website does not require people to log in, and it does not capture any emails, phone numbers, or similar data.
Most people use the Google Chrome browser on their smartphones to search for something and land on your website. Those people don't use the Chrome browser on their smartphone to log in to Facebook, but they do have Facebook accounts, and they also use the Facebook Android or Facebook iOS app on the same device.
How does Facebook match the website visitors mentioned in the above scenario with their respective Facebook accounts and include them in your "website custom audience?"
PS: Facebook Pixel uses cookies to track users in a browser, but the FB Pixel javascript code can't share cookies between the Google Chrome app and the Facebook app on a smartphone due to sandboxing of apps.