Did you come up with the requirements and specs? Did you do all the UX design? Did you write the backend? Did you do the QA testing? Did you get feedback fr

Did you come up with the requirements and specs? Did you do all the UX design? Did you write the backend? Did you do the QA testing? Did you get feedback from users and change the design based on their feedback? If you did all of those things then you can say "I built it", if not, that you can say I was part of the team that built it. A major part perhaps, but a part.