The glasses are used as an Android secondary display. They get power etc. from your phone, and they take about 15 seconds to boot up.
A successful boot-up looks like this;
This is a successful boot. You can then run Snapdragon Spaces applications.
If you launch an application that expects to use the A3 but haven't booted up the glasses when you launch the application, then the app will likely fail when it doesn't immediately see the secondary display and you'll either get a blank screen or see the phone display mirrored on the A3 display.
An unsuccessful boot will deliver a slowly flashing message. You should check for updates (see the getting started page). If everything seems updated, and your software version info from the Companion App matches what the getting started page does, but you still can't get to step 4 or you can't run one of the sample applications, you should then contact Spaces Support.