Merge two Excel or CSV files into one

No. This page reads your file inside your browser tab, and nothing is sent to a server.

Excel workbooks (.xlsx) and CSV files, mixed as you like, up to 20 at once, 50 MB each and 100 MB in all. Drop one workbook to put its sheets under one another. You download the first file's format, as values. Have an old .xls file? Open it in Excel and save it as Excel Workbook (.xlsx).

Add rows lines columns up by their header, ignoring capitals, spaces and underscores, so First Name and first_name become one column. A column found in only some files is kept, empty in the other rows, and the page names it. A last column says which file each row came from.

Each row of the first file looks for a row of the second file with the same value in the column you picked, ignoring capitals, spaces and punctuation. Every row of the first file is kept, in its order; a row with no match gets empty cells. If a value appears twice in the second file, its first row is used. Click Swap to keep the other file's rows instead.

Yes, with Power Query: on the Data tab, click Get Data and load both files, then use Append Queries to add rows or Merge Queries to add columns. Merge Queries keeps only matching rows unless you set Join Kind to Left Outer.