This Ableton tutorial covers how to apply volume fades and crossfades to audio clips directly in the Arrangement view. Fade handles sit at the start and end of every audio clip, and dragging them toward the opposite edge creates the fade. Automation mode must be toggled off for these controls to be accessible.
Once a fade is in place, a slope handle lets you shape the curve from linear to convex or concave. For adjacent clips, dragging from one clip into its neighbor creates a crossfade, which carries its own slope handle for the same kind of curve adjustment.