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

Made up blobs on the lines, tried playing it, replaced some blobs. Tried playing the idea further until found something nice, put the appropriate blobs on the lines. And so on.

Except Bach, who started with a phrase of blobs and said “Now let’s write it backwards. And now upside down.”