Like many pieces of hardware, it is not just the physical components that drive our experiences, but the software or firmware, and this is no different for custom mechanical keyboards. While there are many options out there, a relatively common and popular firmware I have seen for such builds is QMK, which stands for Quantum Mechanical Keyboard. It is an open source project written in C and has quite a number of features including RGB LED control with numerous special effects, macros, the ability to create custom keycodes, mouse commands, and, perhaps most importantly, up to 16 keymap layers. As opposed to keymap profiles, these layers can stack on top of each other and with the KC_TRNS code for 'transparent,' a key press can be passed down to the next layer.