Sorting starts at the row of the selected cell, so a header row stays put. If the range contains formulas we leave it alone — their references would end up pointing at the wrong cells.
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Formulas stay formulas in the file; Excel recalculates them as soon as you open it. Type for example =SUM(A1:A10) or =IF(B2>100,yes,no) — both comma and semicolon are accepted.
An .xlsx you open is not rebuilt: the original file is kept in full and only the cells you touch are rewritten inside it. Charts, images, pivot tables and cell formatting therefore stay. If you change the structure of the sheet (inserting rows or columns, sorting, merging, column width), that is no longer possible and the file is rebuilt after all — the tool says so when that happens.
Open an Excel file (xlsx, xls, csv or ods), edit the cells in a spreadsheet grid and save it again as .xlsx — no Office needed, 100% in your browser. Formulas are preserved and keep calculating when you open the file in Excel afterwards.