Hello, is it possible to create a site, in which once folks “sign up,” they get taken to a user-area which only they can view (must be logged in). Example: a site where when you login, you have your own calendar, notes, etc – and nobody else can see that but the logged in user. I assume it would need to be configured as a multi-site, but I don’t want random users to have access to any other users’ pages/calendars/posts/widgets etc. To that point, each user would need to have a unique calendar, etc. Does this make sense?

Yes, you can.

Through user subscription and member management features you can configure access to specific pages based on their profile.

Explore Admin Zone -> Security -> Usergroups  and Admin ->Setup -> Usergorup subscription.

There is a tutorial video in the official YouTube channel, though it is for PayPal, the approach is similar.

Check it below: