Hi VAXman,
Welcome to this Apple (user-to-user) Community!
Re: "Confusion with two rows of digits in new Calculator"
Wondering which "mode" you have the calculator in:
Apple Support's onllne article: Choose the right mode in Calculator on Mac - Apple Support
Eg: says:
"Calculator has several different modes that are each useful for different kinds of calculations and conversions."
(The modes are all listed individually in the above article:)
Eg: Article says:
- ":Go to the Calculator app
on your Mac. - Choose an option from the View menu:
Tip: To quickly access other modes in the basic, scientific or programmer calculator, click 
Trusting the available resources will help you ease into mastering any relevant new capabilities in Calculator..
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Resource for MacOS Sequoia: Calculator User Guide for Mac - Apple Support