If you look at the first Secondary Dominant listed, it's a Eb7b5 chord, but the base note is an A. So, they are not even listing that as an A chord?! But rather Eb - which is the 4th chord in the scale. What is going on?? Do they just make this stuff up to confuse the heck out of you??? AUGH! It's a conspiracy, I tell you! A Conspiracy!
It made me so broke in the brainpan that I walked away from the theory book for a while. However, I knew it was just a case of finding the right tutor. So, back to youtube to find such a helper. I really was desperate and found more confusing teachers along the way. Even their examples hurt my wee brain!
Finally, I found Mr. Matthew Barrette. I saw he had 100 likes to his video and not a single dislike, which made me think, "hmm... He must have a good explanation." And indeed he has the best, though I can't say I enjoy the jazz music in the background. I find it distracting. However, let's not quibble about the small things. Mr. Barrette helped me understand after all. So, here's his explanation. Thank you, sir, my hat goes off to you.
I still don't understand why that chord is considered an Eb chord when there's an A in the base. The A is what makes it a V of iii secondary dominant. If you could break this down for me, I would sure appreciate it. Now that I get the concept of Secondary Dominants and Leading Tones, it would be nice to understand why chords can be called one name when they seem to be another.
Addendum (Later that same day):
Well, turns out that the example above (the picture) was only pointing to the single base note as the Secondary Dominant and not an entire chord (as the video shows). Houston, we have liftoff!
And upon further thought, it seems that there's a reason why you would call that chord an Eb, even though the A is the base. Because then it would be an A(b5b9add13) and WHO THE HECK WANTS TO READ THAT?!? An Eb7b5 is actually easier to read. Jazz Theory is crazy. I love it!