A workbook can hold many sheets: inputs on one, months on others, a summary on the last. A formula can use any of them.
To point at another sheet, write its name, an exclamation mark, then the cell: =Inputs!B2. You rarely type this: start the formula, click the other sheet's tab, click the cell, and press Enter. Excel writes the reference and takes you back.
If the sheet name has a space, Excel wraps it in single quotes: ='North Region'!B6.
To add the same cell across monthly sheets, add each one: =Jan!B5+Feb!B5+Mar!B5. Excel also has a shortcut for a block of sheets, =SUM(Jan:Mar!B5), which this practice sheet doesn't support yet.
On long sheets, View > Freeze Panes > Freeze Top Row keeps the headers in view while you scroll.