Your kid will get an email invite from their teacher with a link or a code they use to join a class. They'll be directed to classroom.google.com, where they log in (using either their school-created account or their personal Gmail account) to access the class, their teacher has added them to. Each class is private to the people the teacher has personally invited, including the students enrolled in that class and other teachers.
Your student will receive an email when the teacher posts an announcement. These emails come through your student's email account, not in Classroom. Classroom doesn't alert you when an assignment is due.
If your students are younger, it's probably a good idea to have some familiarity with Google's G Suite so you can help your kid upload documents, check the calendar, and do other tasks. Older kids may not need any help. Google Classroom is designed to be easy to use, and there's lots of online help.