Where is my US stocks account held? Who is responsible for custody and clearing?

Your brokerage account is managed by Viewtrade Securities (VTS). VTS works with Apex Clearing Corporation for trade clearing and custodian aspects of user accounts. 

VTS is registered as a securities broker-dealer with the US Securities Exchange Commission (“SEC”), (SEC# 8-51605) and is a member of the Financial Industry Regulatory Authority, Inc. (“FINRA”) (CRD# 46987); and Securities Investor Protection Corporation (“SIPC”).