Apologies for the delay.
The query functions are for specific tables. You cannot join tables directly in the API. However, in reality, the tables you see in Method are actually Views behind the scenes, and you can add Linked Fields (via Customize > Tables / Fields) that show values from other related tables. For example, on ItemReceiptLineItem, you can expose fields from ItemReceipt table, like the ItemReceipt.Vendor. Pretty safe to say that if the ItemReceipt.Vendor is blank, there is no association between the ItemReceiptLineItem and an ItemReceipt.
Thanks
Chris