Brep join or Solid Union

I would like to join two breps:

  1. Box
  2. Trimmed surface by ‘isotrim’ → ‘extruded’ breps

I would like to calculate ‘total surface area’ so I tried ‘solid union.’ However, it did not work. ‘Brep join’ did not work, either.

After I baked, it remains as each separate boxes.

How can I join them all together - one big box and trimmed boxes?
Please help.
Brep (14.1 KB)