This isn't a question of ownership in the sense of a private property. It is about how you interact and respect and professionalism. First of all, a larger change to code a teammate wrote has a ring of saying: it was badly written. So this is partial a matter of social interaction. If the code is badly written, it needs to be fixed, but one should be reasonably polite about that. Usually, the better way of doing this is having a quick chat with that person where you explain why and how you think the code should be changed, before you just do it. Also to pass on any insight you have about the code, the creator missed.

On the other side, there is the big elephant in the room: unless we are talking about rather simple code, the code perhaps was written as it was for a reason. This often enough isn't apparent on the first glance. So a refactoring without checking with the author always bears the risk that you introduce a bug or at least get into the way of planned changes, which would be incompatible with the refactoring.

So there are many reasons to quickly check with the author before doing significant changes. If the proposed changes are good, then the author will be happy about you doing them, if not, then its better to have talked about them before doing them. And in any case, it is good to have communicaton about major changes.