I’m a DIY beginner and I plan to make a midibox SID with the complete control surface (step C).
So far I think I understand pretty well the construction process but I have a (probably silly) question : what kind of switches should I use for the control surface ?
You want momentary closing switches. What kind you use is entirely up to you. Typically “tactile switches” are used as they’re easily available and way cheaper than nice push buttons.