Need to understand how my seat post works

Hi everyone, first post ever on reddit and I'm french so please excuse me if I don't get everything right.
I have a problem on a look 795 aerolight that I bought second hand a few months ago.
It has a di2 system and I needed to recharge the battery. First I thought I needed to remove the battery located on the seat tube in order to charge it but later I realized that I can could just charge the battery from the charging port on top of the frame.
Anyway the problem is that when I remove the seat post, I discovered a weird rope system that maintains the seat post in the seat tube.
It's now disassembled and I have now idea how to fix it because I was only able to remove half of the seat post and the other part is stuck on the frame and can't be removed even though it's supposed to.
Does anyone have any idea of what I'm supposed to do on this case?