I've always thought of the idea "When can you say you know how to play the piano"

If I know all the correct keys to hit for 1 song? 2,3,4 songs...

If I can read any sheet music and play

If I can come up with my own music

For example my brother can sit down at the piano and play the Fur Elise. He cannot read sheet music and I'm sure it's not done technically right all the way through. But can he play the piano? When does he get to say he can play the piano? 15 songs? Guitar players do not have to learn to read sheet music and if a person could play 15 songs on guitar you'd probably would say they could play the guitar fairly well.

I can bake cookies from my favorite recipe, I just don't understand the food science behind it. Just like not understanding all the music theory behind a piano piece

Kind of like if you gave a master piano player a piano with a single broken key and told them to play a song. (With my limited music understanding) they could play the song using a different "key" to avoid the broken key.

A master chef could do something similar where it's like "I only have this type of flour, I better adjust..."