Hello,
The videos are optimized for people who are buying kits, since there are probably about ten times as many people building from kits than self-sourcing parts for this project. But we do publish all the info needed for self-source makers as well.
If you are self-sourcing the parts, the information on constructing the electrical system is here:
It is linked from the instructions in the Bill of Materials section. See Bill of Materials, right at the start of section there's "NOTES" and this link is in bullet number 1 right there.
If you bought a kit, these parts are pre-assembled for you. The online instructions do describe how to attach the switch into the robot body, etc. There's even a specific video that shows how to put all the electronics into the electronics caddy and set the battery holder on top.
Hope this helps,
Steve P.