Karabiner-Elements

Remaps any key to any key, builds layers, and applies different rules per keyboard.

What it does

Karabiner-Elements sits below the application layer, so its remappings apply everywhere including login and recovery-adjacent contexts. Beyond simple swaps it supports:

Complex modifications — conditional rules with JSON definitions, per-application or per-device.
Layers — hold a key to turn the alphabet into arrows, media keys or symbols.
Simultaneous keys and tap versus hold distinctions, so Caps Lock can be Escape when tapped and Control when held.
Per-device profiles, which is what makes an external PC keyboard and the built-in one behave consistently.

Karabiner-EventViewer ships alongside it and shows the raw key codes a device is actually sending — the first stop when a key does not do what a rule expects.

Notes

It installs a driver extension and needs Input Monitoring permission; on Apple Silicon expect one approval and a reboot at install. Rules live in ~/.config/karabiner/karabiner.json, which is worth keeping in a dotfiles repository.

Alternative to

Alternative	Type	Trade-off
Hammerspoon	Open source	Lua scripting for the whole system, keyboard included
BetterTouchTool	Paid	Broader: trackpad gestures, window snapping, remapping
Keyboard Maestro	Paid	Macro-first rather than key-first, and very powerful
Kanata	Open source	The same layer model as a cross-platform CLI daemon
QMK or ZMK firmware	Open source	Remapping in the keyboard itself, so it follows the hardware
System Settings → Keyboard → Modifier Keys	Built in	Enough for Caps Lock to Escape alone