What is the difference between Union, Join and Append Fields?

Union stacks datasets vertically — same columns, more rows. Configure it to match by field name, not position, or differently ordered columns are silently misaligned.

Join matches rows between two inputs on a key, giving matched and unmatched outputs.

Append Fields attaches every source row to every target row — a Cartesian product. It is intended for adding a single row of parameters to every data row, and multiplies your data if the source has more than one row.