Here's a simplified explanation of who can see your posts and submissions:
Activities: Activities that your program administrators set up appear in My Feed. If an activity has a lock icon, it's private, and only program administrators can see your responses. If it's public, everyone can see your responses.
Polls, Quick Questions, and Quizzes: Your responses to these are private and not visible to other community members.
Discussions: Only members of a specific group can see discussion posts in that group, whether the group is public or private.
Comments: If you comment on a blog post or another member's submission that is visible to the whole community, your comment will be visible to everyone. If you're in a private group and have private profile visibility, your name will also show up next to your comments or posts.
Non-group members can't post in any groups, whether they are public or private.