Most residential Polycade arcade machines were configured to launch directly into Steam at startup. When you install AGS, this will not automatically change this setting, but there is a way to AGS the startup program instead of Steam:
First, quit out of Steam:
You can use your trackball to quit out of steam by scrolling to the Power icon (in the upper right of the Steam window) and then selecting Exit Steam
Next, launch AGS:
Use the trackball to move your cursor search bar. Connect a keyboard to your Polycade and type "AGS." You should see "Polycade AGS" as an option. Click that to open AGS:

Set your startup program
In AGS, go to:
Settings > General > Launch on Windows Startup

Toggle that ON.
Next time you restart your Polycade, it will boot directly into AGS instead of Steam