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

Before audio recording existed, the only place ine could hear a symphony outside of a music hall was inside their own head. Being a composer meant studying to learn how to transscribe what the imagination conjures, so that a group of musicians could musically manifest that which, uo to that point, existed only as an idea.