This Ableton tutorial walks through every content category inside Live 12's built-in Library, giving you a clear map of where different asset types live before you start a project.
Sounds holds instrument presets like synths, guitars, and pianos, while Drums covers percussion instruments and Instruments gives you Live's built-in synths and samplers in their raw, unedited forms. Audio Effects, MIDI Effects, and Modulators are each separated out, so signal processing, MIDI manipulation, and parameter modulation all have their own dedicated sections.
Max for Live content, third-party VST and AU plugins, Clips, and Samples each get their own category too. Grooves stores rhythmic templates you can drag onto patterns, Tunings holds custom scale files for microtonality or retuning, and Templates gives you access to any pre-configured Live Sets from Ableton's content or your own saved setups.