When I was in my final year of 6th Form which is 18 years old. I designed and built a recumbent bicycle.
From a hands on perspective I learnt how to weld and form wood using a vacuum press bag.
On the design side I learnt about beam deflection. Given a known cross-section, material and thickness it's fairly straightforward to calculate how much it will flex and if it will buckle for a given load.
I used Mild steel for the frame because it's cheap and has a nice failure mode, bending rather than snapping. It was satisfying when I stood in the middle and the deflection was about 16mm and I'd calculated the deflection to be 10-20mm. Although it didn't tell me what I really wanted to know which was would it survive a speed bump! Guestimating the real world forces is the hardest part.
This took up a lot of space and has since been taken apart. I didn't like that it wasn't possible to hop up and down curbs and it handled like a boat.
Things I'd change
Move the rear wheel as forward as possible without it binding on the seat. Making it more compact and handle less like a boat.
Start the design around a straight chainline, each kink adds a significant amount of friction and an additional wear point. It took three complete chains to run the length.