How do you make a workbook that other people will not break?

Separate raw data, calculations and presentation onto different sheets. Use Tables so ranges grow automatically. Never hard-code constants inside formulas — put them in labelled cells and reference them.

Apply data validation to input cells, then unlock only those cells and protect the sheet. That prevents accidental edits to formulas without blocking legitimate data entry.

Avoid merged cells, which break sorting, filtering and structured references. Document anything non-obvious; the person maintaining it later is usually you.