Some things that helped me

Every query response in SQL is a rectangular matrix [1]. JOINs add columns sideways. UNIONs add rows vertically[2].

[1] Which is why tree shaped data is awkward to query in SQL in one round-trip.

[2] From this you realize why the column names have to match to apply a UNION, and why recursion is something to do with a UNION.

And if you get stuck, since it is all about rows and columns, just sketch it out in Excel.

Not using formulas in Excel but just using it as a rows/columns editor.

This makes it visually clearer.