So with open source software more on my mind lately I was wondering - while I get the benefits of transparency and such, how safe is it? If the source code is available to all, isn’t it easier to breach for people (like the recent cookies hack)? If I’d have an open source password manager, would it be easier for people to get my passwords somehow than if I use something not open source? Do I just not understand how software works in general?

And what are other benefits that may be not so obvious to someone not so knowledgable about this?

Edit: thank you all for really insightful answers! Among other things I also learned just how much I don’t know :)