Upvote:2
You can only pass through immigration and thus exit the airport if you can get a visum. Since the Philippines is not on this list: https://en.wikipedia.org/wiki/Visa_policy_of_South_Korea You need to apply for a visum in advance.
Some airports have these "free city tours" for these tours you do not clear immigration and thus does not require a visum.
The fact that you hold a visum for the US does not make a difference for South Korea.