I'm coming from Windows where I got used to write AHK scripts for everything I needed. Very important for my daily work are hotkeys to send special characters. Like here with Windows+a:
#a::ä
I don't like the MacOs "normal" way to create this special characters. So I want to write a script for this. I think that using "Automator" does not work, as it is not running in the background and watching all my keystrokes. Right? Is there any alternative?
I need it to start automatically on startup and it should be active in all apps.
Karabiner is great for re-mapping key presses. It can remap simply keys by GUI and allows you to configure complex remaps with different key combinations within its karabiner.json.
If you need something more sophisticated that goes beyond simple key events, BetterTouchTool can help you out. BetterTouchTool doesn't require to write scripts, all is done by GUI.