About what you asked me in my post, I leave you this link, which is the closest I've seen to a bicycle shop here in Havana. They don't sell things.
It's a workshop and bike rental. They offer mechanics courses as well. They have an Instagram account.

@dahpilot/new-way-to-discover-havana

On the subject of changing the chain... mmmm, yes. Fortunately I don't have to change the one on my new bike. A friend of mine measured it and he says it's not worn enough to change it yet.

Today I finished cleaning the bike.

! [Before and After]