On step 8 of page 76 I do not understand what holds the two sections together. The "LEGO Flat Silver Technic Axle Joiner Perpendicular with Extension (53586)" has a smooth bore where it attaches to the "LEGO Medium Stone Gray Technic Cross Block 1 x 3 (Pin/Pin/Pin) with 4 Pins (48989)". And directly under the first part is a Technic pin (part# 4211815) in yet another smooth bore opening (part# 4526985).
The "LEGO Flat Silver Technic Axle Joiner Perpendicular with Extension (53586)" smooth bore piece is secured to a technic pin on piece "Technic, Pin Connector Perpendicular 3L with 4 Pins (48989)" installed in step 5, with the relevant pin marked in step 7 in my marked up directions below:
The smooth bore piece connects to the pin holding them together, with the lower axle connection preventing rotation.