Before the motors can be attached to the Booms, they need to be connected to the Motor Mounts, which are one of the sacrifical parts of the ELEV-8 v3.
Sacrificial parts are intentionally engineered to fail under excess mechanical stess. They are engineered to fail first, thus protecting other parts of the system, just as an elecrrical fuse protects an elecrical circuit. In this case, the motor mount is engineered to to break before the motor is damaged in most (but not all) crashes, since it is less expensive and easier to replace than the motor.
Tip! For your reference, wherever hardware is included in a Parts list, just click on the item link to open this image in a new window.
Links
[1] https://learn.parallax.com/sites/default/files/content/ELEV-8/v3-Assembly/v3-HardwarePackSingles-Labeled.png