Piazza is the main communication channel. If you have any questions or concerns, post on Piazza. You can do either public or private posts (annonymously if you want!). If others could potentially benefit from your question, consider making it public. Never send your homework/project solution code in a public post.
Gradescope is used for assignment submission and handling. You make all your submissions on Gradescope and recieve feedback and your work in there. Regade requests can be made and are processed on Gradescope as well. You get an invitation to the course Gradescope in the first week.