For example, a teenager is asked by his friends if he watches porn(let’s pretend the teenager does watch porn), and the teenager replies no because he’s too embarrassed to tell them. Is this a sin?
Lying is Haraam. Find better friends who respect your privacy and who do not interrogate you. I am not sure anyone who acts this way can be called a friend.
One needs to please Allah instead of the creation. In any case one should never confess or advertise any immoral acts or sins they may have committed to others. Seek forgiveness from Allah and spend your time in pursuing useful knowledge that will help you in the hereafter.