Let's check out these logic board sandwiches. For reference, the US version is on top in these images, EU below. (Don't read into it—that's just how the chips fell.)

First, the similarities—the US and EU upper sections are identical and have the same spread of chips on both sides.

The lower sections are a bit more interesting. (Note: SIM readers are the same, but we only desoldered it from the US version.) Here's where the US board starts accommodating all that mmWave tech:

An extra socket, which connects to the front mmWave antenna mentioned in the previous step

A Murata 1XR-482 mmWave front-end module

A Qualcomm SMR526 intermediate frequency IC, working in conjunction with Qualcomm's SDX55M 5G modem

One more mmWave antenna right on the board

A flex cable soldered to this patch, which connects to the side edge mmWave antenna