Spreadsheet files confuse people because two very different things share the word "spreadsheet." CSV and XLSX are not two flavors of the same thing — they're a plain text list versus a real document.
CSV is a comma-separated text file. One row per line, one value per column, nothing else. That's its strength: every system on earth can read it — databases, scripts, old machines, other companies' tools. Its weakness is everything a CSV can't hold: multiple sheets, formulas, styling, dates done right, or merged cells. If you're feeding data to software or importing it somewhere, CSV is usually what's wanted.
XLSX is a true Excel workbook. It keeps multiple sheets, live formulas, number formatting, and styling. Use it when a human will open and work in the file, or when the data has structure a flat list can't express. The weakness: not every system accepts it, and a .csv export from Excel often mangles long numbers and dates on open.
Decision: "Will software consume it?" → CSV. "Will a person edit it, or does it need sheets/formulas?" → XLSX. To turn a plain export into something someone can open in Excel, the CSV to XLSX tool builds a genuine .xlsx in your browser — the first row becomes the header, values land in real cells, nothing uploaded. To go back, the XLSX to CSV tool flattens a sheet and writes formulas as their calculated values.
Quick path: pick by consumer (software vs human), convert with the matching tool, and keep the original untouched. Your payroll, customer, or grade data stays on your machine the whole time.