where does the info come from?

You can tell the user ID of the person who did the edits and when they did them. On the left side of the map is a layers icon. Clicking on that will show a check box for data. You can see the various items and select individual ones to get more information about it. For example Cowan Park shows, among other things:

History for Cowan Park Details

Edited by drnoble at 2012-12-16T15:58:50Z
Edited by Filling in the blanks at 2008-04-27T08:42:06Z
Edited by Filling in the blanks at 2008-04-26T16:37:21Z
Edited by Filling in the blanks at 2008-04-07T08:17:01Z

With JOSM (and maybe other editors but I mostly use JOSM) the object's edit history is also available and you can look at things like the check-in comment which, for the latest "drnoble" edit, is "Around Cowan Park, Barrhead from GPS survey/bing". So there you have it - GPS combined with Bing imagery.

There are web tools for looking up information about the contributors and where else they've made contributions. You can then contact the contributor and clarify where they got the information from if they haven't been as descriptive as you'd like with the "source" key value.