The Rive editor has two distinct modes, Design and Animate. As you switch between modes, the interface changes to show the appropriate tools and options available.
Use this mode to create and rig your graphics to set them up for use in Animate Mode. Design Mode is the default mode for any file that doesn't have any animations created. The mode exists because Rive allows you to attach multiple animations to a single artboard, so you need a place to set up and create those graphics.
Use Animate Mode to create all of the animations for your artboard.
When you switch to Animate Mode, the UI updates to display a timeline with a list of animations for the currently active artboard. The Inspector also updates to show key buttons next to any property that can be animated.