How this FILTER formula works
The combined mask is true for the Widget West and Gadget West rows. FILTER returns those two complete rows as a 2-by-3 dynamic array.
The top row above is the whole formula. Open any argument that holds a calculation of its own to see the values it used. Excel works out those inner parts first, because the function around them needs their results. Edit a cell in the grid to see every value update.
Try your own formula and data
The sample spreadsheet is fully editable. Change the formula or any cell it references. The intermediate values recalculate in your browser.