Some References for Mechanical subsystems:
An FRC Mechanism Encyclopedia https://www.projectb.net.au/resources/robot-mechanisms/
Tapping Basics: https://www.youtube.com/watch?v=r6Ijd9o-C10 - The first part of the video covers tapping which we do very commonly. The second part covers dies (used to put threads onto a rod) which we rarely use.
CNC Basics: https://www.youtube.com/watch?v=YBGqknN3gGs
Onshape Intro to CAD Learning Pathway: https://learn.onshape.com/learn/learning-path/introduction-to-cad
FRC Specific Onshape Getting Started: https://onshape4frc.com/getting-started
NASA Robotics Alliance Project Design Guide (Sections 2-5, 7 & 8): https://robotics.nasa.gov/nasa-rap-robotics-design-guide/