SQL joins as venn diagram

Question

I've had difficulties comprehending joins in SQL, but I think this illustration might help. The issue is that I'm not quite clear about it. For instance, the connect in the image's upper right corner, simply paints the overlap between A and the entire B circle red. The graphic gives the sense that circle B is the main focus of the SQL statement, but the statement itself, which begins with A (select from A, join B), gives me the feeling that A would be the statement's primary focus.

In a similar vein, the figure below only contains information from the B circle; hence, why is A even mentioned in the join statement?

Can someone provide an additional explanation regarding the representation of each SQL image, explaining it starting from the top right and moving to the middle

a) the reasons why a join would be required in each circumstance, such as when just one or the other, but not both, of the two sets of data acoloredred.

b) and any further information that might help to explain why the image accurately represents the data.