"Swiping left" on a row in the Focused Calculator or Function Calculator lists of a custom calculator show an "Share Action".
Tapping this action will present a standard iOS Sharing Action sheet. This allows you to create a copy of the calculator and share it using Message, Mail, Notes, Save to Files or AirDrop.
Saving your calculator to Files allows to to make a backup copy of your custom calculator. You might want to make a folder in your iCloud drive for CALC 1 Focused and Function Calculators.
This also allows you to send a copy of the calculator that can be loaded into CALC 1 on another device.
From your iCloud Drive, a message or email or a note, a CALC 1 calculator will have the name of the calculator or function ending with the version number with a "calc1js' extension. From that application you can share the file to CALC 1 on your device. Tap the file and use the share action for that application. Choose the"Copy to CALC 1" action.
If you do not see the calculator in the calculator list, go back to the main menu and select the list to refresh it. If you still do not see it check the History List for a item. The new calculator version must be higher than an existing calculator it is replacing so keep the version number of your custom calculators up to date.
Before a calculator is deleted, a copy of it is saved to History. You can use the "Restore" button of the History Item to restore the calculator.
CALC 1 only keeps about 200 History Items, so it is a good idea to share your custom calculators to your iCloud drive.