Spaceport.academy is a free online learning website focusing on space exploration and space technologies. Clicking the Mind Map link reveals how the lessons are organized by track: Space Science, Biology in Space, Space Applications, Robotics, and Engineering. The Engineering track contains numerous lessons relevant to cubesats.
When you register, please consider joining the QMR-KWT Space Challenge for the opportunity to submit code to run on the QMR_KWT cubesat. I was the 136th person to join (as of Dec 2021). And after completing only 2 lessons, my ranking raised to 39. This indicates that you have a strong possibility of being selected should you complete the required 20 lessons. Details available at: https://www.orbitalspace.org/cis