If I ask someone how they managed to find something, I am certainly expressing an idea of being impressed with their discovery (this could be positive or negative). Let's say someone finds a photo of me doing something embarrassing and I thought I had hidden that photo from social media. I might react "Oh goodness, how did you manage to find that?". I could also say "how did you come to find that?" but it gives the idea that I'm not so moved by, or that I don't mind, the discovery. Let's say you are in a more formal setting, and show some statistics to your colleagues or boss about something. I would expect to hear "How did you come to find that statistic?". Additionally, perhaps you could see the phrase with the verb "to come" as a more neutral phrase, whereas "to manage" comes with the nuance of surprise or being impressed by some amount of effort that must have been behind the action we managed to do. There's always layers to everything, and I'm sure I could think of many cases where I could use both of the phrases for the same situation, or where one person might choose a different response than I would.