Could I ask why you would like to learn this and what part of knowledge you think it would expand?
The pop arrangements one buys at the store are ****. It has always been this way, I have a lot of **** arrangements from the 40's and 50's.
Real transcriptions of what artists actually played, cost $80 (20 years ago) and up.
For those of you that think music stopped when Rachmanoff died, you have no clue. There are still talented musicians out there but few write for solo piano. You can make so much more money with a couple of guys on guitar and a bass player. Doesn't mean I'm going to play guitar - wire strings make my skin bleed. I'm not referring to Lady Gaga either. My favorite bands these days from oldest to newest, Arcade Fire, Gnarles Barkeley, White Denim, TV on the Radio. Actual melodies are being created, actual stories are being told to music. Not all baby boomers got stuck on the Beatles and the Eagles either, however much satellite and mega-corporation radio pounds those into our brains.
I'm learning to play by ear starting in my sixties. My piano teacher refused to teach me how when I was 12. I wanted to play "riders in the sky". She couldn't imagine me wanting to play "that old chestnut by moldy old Vaugh Monroe". So why did it get a two minute airing in top movie "Blues Brothers 2000"?
First learn chord structure. I did that age 12. Then train the ear to hear the different ones. I've been doing that the last few years - many are required to do this now in UK music programs and the pop music programs at US colleges like Belmont (Nashville).
After you can hear the chords by number with any key as the root, try to pick out melodies over them. Play along with the record. I'm enough into it, my hands are starting to feel melodies as I listen to the radio. The more tries you make, the more accurate you get at picking out intervals from the root. Practice makes perfect, as some Gregorian chanter said.
You don't have to be born that way to acquire this talent. I envy those that were, but few of those can play Moussorgski"s P@anE as I can do. Listen to the mess Keith Emerson made of it. I'm trying to play both ways, Why watch TV or travel to the beach or take recreational drugs? Music is so much focused and more rewarding, and costs almost nothing to practice. Do it yourself, nobody else can please your psyche as you can.