I recently answered a question with a bounty offered by "Community". How are such bounties awarded? Who decides which response gets the bounty?
As explained in some older posts, the fact that the bounty is owned by the Community user means that the user who originally offered the bounty have deleted their account. See: Bounty by Community? and How Does the Community User Set Bounties?
You can find similar past bounties in the bounties tab of the Community user profile's page. If you look at timeline or revision history of some of those question, you can still see who offered the bounty originally - the user is shown in the usual way for deleted accounts, so you'll see something like "user123456".
You have also asked which answer gets the bounty. There are rules how the bounties are handled if the user does not select any answer. As far as I can tell, those rules are applied also when the user in question no longer exists. How does the bounty system work? The details can be found in the FAQ post How does the bounty system work? (current revision) - see the section: "What happens if I feel my question is still unanswered? / What is automatic awarding?"