macOS supports emacs-like bindings natively: Ctrl-A/Ctrl-E for line start/end, Ctrl-K to kill, Ctrl-Y to yank, Ctrl-F/Ctrl-B for forward/back, etc.
They are defined in /System/Library/Frameworks/AppKit.framework/Resources/StandardKeyBinding.dict. You can create ~/Library/KeyBindings/DefaultKeyBinding.dict to augment or override those.
Anything built on AppKit that uses the standard text system (NSTextView, NSTextField, and the field editors that back them) supports those bindings “for free”.
For non-native apps, Karabiner Elements⁽¹⁾ is the answer, using a config like this⁽²⁾.
They are defined in /System/Library/Frameworks/AppKit.framework/Resources/StandardKeyBinding.dict. You can create ~/Library/KeyBindings/DefaultKeyBinding.dict to augment or override those.
Anything built on AppKit that uses the standard text system (NSTextView, NSTextField, and the field editors that back them) supports those bindings “for free”.
For non-native apps, Karabiner Elements⁽¹⁾ is the answer, using a config like this⁽²⁾.
⁽¹⁾ https://karabiner-elements.pqrs.org/
⁽²⁾ https://github.com/pdelfino/karabiner-config