When all resources are loaded and the AR module of WebXR is available, you can enter the AR experience by pressing the glowing logo. The first time you do this, the browser will ask for permission to start an AR session. Once you're in AR mode, move your device around until a recticle is shown (see Move device below). When you then touch the screen, the model will be placed at the recticle's location. The recticle will now be hidden until the model is removed again.

The following UI elements are available:

Exit AR: Press this to exit the AR experience.

Remove model: Removes the model. The recticle will be activated again, so the model can be placed at another location.
This button is only enabled when the model has been placed.

Toggle scale: Toggles the model between normal and full size.
This button is only enabled when the model has been placed.

Umbrella: Take out or put away umbrella.
This button is only enabled when the model has been placed.

Whistle: Start or stop whistling.
This button is only enabled when the model has been placed.

Move device: This icon indicates that the user should move their device around to help build the underlying environment map.
For AR to work, the browser's underlying AR framework needs to understand the real world near the user. It does this by progressively creating a 3D map while moving the device through the environment. To expedite this, the icon is shown to encourage the user to move their device around. A well lit environment also helps this process.
Note that if you point or move your device to a new part of the environment, then the icon might be shown again while the environment map is being expanded.

Since the AR module of the WebXR standard is fairly new, your browser might not yet support it; or you might have to enable it explicitly. Chrome v81 and up should support it out of the box.