How does Power BI know to automatically pull in data from related SQL tables in a query?

Sorry, I'm not sure how to better phrase that!

When one goes to get data, and goes into a SQL database - at least on the database I use, the table you pull, will often have these additional columns where Power BI is essentially offering to auto join and pull in data from related tables.  See screenshot. 

Just curious - how does Power BI know to make those connections? 

We use SQL server here (a Microsoft product) - i don't know if this functionality appears in all flavors of Power BI - SQL interactions.