Where does the Sites column information come from?

Hello.

I have a multisite network.

In the Network Admin > Users screen, it lists all the users on the entire network.

In the Sites column, it lists the sites associated with each user.

Where does this information come from? Is there a way I can access it programatically (so that I can set up redirects based on the site listed here)?

The reason I’m asking is that some of the users do not have a primary_blog set in their user meta, but it still shows the correct site in the list here.

So I wonder where this information is coming from.
Thank you.