This question is really offtopic here, but since you pledged some bounty you deserve at least a short answer. :)
I don't think there is a way to search for tags on osm.org - it is basically only one frontend of many to view the data. It is not completely clear what you want to do with the replied data, but I presume this would suit you. The Overpass API is the most common way to query the database for specific data and this website is one interface using it (but you could do the same on a command line against a local dump of the database for example).
The data is similarly maintained as wikipedia: volunteers that get data from the ground by surveying, using open government data and other suitably licensed sources (e.g., Bing aerial photos have been provided).
Like on wikipedia every with an account can edit the data - thus there exist similar problems like individuals and corporations trying to push their interests. I know of various incidents regarding Israel and China military installations, private owners of forests with paths that they did not want to be on any map etc. but in general vandalism and especially simply bad volunteers are bigger problems to OSM's data quality than governments.