If playing piano by ear is something you aspire to either learn to do or become more competent at, consider two levels of familiarization that you want to get yourself to experience:
1) Learn the 7 diatonic chords in every key
or, for starters…
2) Learn the 3 most essential of those diatonic chords in every key
Start with one key, then two, and continue from that point…
There’s no magic to it. Let’s say there is a favorite song you like to listen to and feel inspired to learn that song on the piano…
The one thing you can do to make the process easier is to set your “radar” to recognize where those three essential chords are being played in the tune. These chords are often referred to as the three “primary” chords of a key.
You want to know them. Even if you are brand new at learning and don’t know how to play a chord, you can take a step-by-step approach… yes, learn those chords “one letter at a time” by using a guide such as this one
It should be noted that this guidebook assumes nothing in terms of what you know. It’s one you’ll want to have propped up on your piano or keyboard stand as you follow in steps. Essentially, it’s for beginners or those who have played via reading but are totally new to how a chord is constructed.
Follow the suggestions all the way through and you’ll have a handle on those 3 primary chords, which are the ones you’ll want to be listening for in those favorite songs of yours.
It doesn’t matter if you are wanting to play “Happy Birthday” or “Let It Be” by the Beatles… it’s still those three chords you want a handle on. So many people bypass/ignore this basic step of learning when it comes to playing by ear. If acknowledged, the door is opened for unlocking the mystery to those more complex tunes… but, as the saying goes, one step at a time.
PLAY WITH PASSION!