The calculator starts you as a guest. Those plans stay with that guest session and are removed after 30 days without activity. Choose Save, then create an account with your email and a password, or sign in if you already have one. You can also export a JSON file if you just want a copy on your computer.
After you save
The same email and password open your plans on another device. Sharing is only available after you convert the guest session to an account. If you sign in to an existing account, guest plans from that device are added to it.