How are columns matched when stacking?

By header name — the result has the union of all columns, and files missing a column get blanks there.

Can I join like VLOOKUP?

Yes — a left join of two files on a key column you pick in each; unmatched rows are kept with blanks and honestly counted.

No — merging happens entirely in your browser.

Combine spreadsheets into one table with header-aligned stacking, or left-join two files on a key column. Private, in your browser.