Eli5 How did classical music composers memorize their songs back then after creating them?

Question

Let’s take for example the nocturnes of Chopin.

My preferred one is nocturne n. 20

He certainly didn’t have studio machine to record their sound AFAIK

How then did he memorize and write down everything to remember later?

Did he start playing the piano, write down the notes, continue playing or did he memorize the whole song by heart without any writing process?

Sorry 4 my bad English btw. Not a English native speaker.

EDIT : thanks everyone for the explanation.

In: 157

Answer 1

An author who writes a book doesn’t memorize it either beforehand. Writing music can be similar. Chopin may have an idea of the overall melody, write a piece of it, play again, change a bit, then starts the work of designing the variations and the harmonics. Again, trials and errors. So, it’s not like he played it entirely in his head and then rushed to write it.