Using software such as QMK you can program these custom boards to do anything. To answer your question all keys are accessible by turning whichever key you like

Using software such as QMK you can program these custom boards to do anything. To answer your question all keys are accessible by turning whichever key you like into a modifier when held down so you can access another layer. It's not that bad when you realize through your entire life you do the same thing with SHIFT to capitalize letters for example. All of these small boards are just an extension of that concept.