"What key to press" isn't the best way to think about it. Music is full of patterns, idioms, and so on, and we practice those too. That's why beginners are taug

"What key to press" isn't the best way to think about it. Music is full of patterns, idioms, and so on, and we practice those too. That's why beginners are taught scales and arpeggios, and to get into rock or jazz, you learn their chords, progressions, etc. In terms of what it's like to be able to do it, I'm a competent church musician, and generally I prefer to pick up songs from recordings than from sheet music – I can generally listen 2 or 3 times and then sit down at the piano and do a decent enough rendition. When I'm very familiar with a song, I quite like to rearrange, reharmonise, etc. This has required both study and deliberate practice – I look for opportunities to use particular reharmonisation tricks, for example. Start with stuff you like, and play along to recordings. Choose a level of difficulty that stretches you a little, and keep practising until it both sounds and feels good. Have fun!