Nothing appearing can mean the cable, the USB mode, developer settings, or a firmware that never exposes the interface. The order matters, and the first answer decides the rest.
Glasses and standalone headsets are not the same problem even when the sentence sounds identical. Glasses are a USB peripheral that answers HID or a serial protocol; a headset is an Android computer that answers nothing until developer mode is on and ADB is exposed. Asking which one you have is not politeness — it decides the next step.
Glasses that produce no reaction at all are almost always a cable: charge-only USB-C cables are physically identical to data cables and are the most common single cause. If the screen flickers but the checker finds nothing, the video link is working and the control channel is not, which is a different layer again — an interface that never enumerated, sometimes a firmware revision.
A headset that is not seen is usually a setting. Until developer mode and USB debugging are on, the port is a charger as far as the computer is concerned. And once ADB does appear, the very first connection stops at an authorisation prompt on the headset screen — the state people most often read as "ADB does not work".
These checks are read-only and each one eliminates a layer: no USB reaction rules in the cable; a descriptor with no control interface rules in the firmware; an ADB interface that answers rules in the cable, the port and the debugging setting all at once. That is why the walkthrough asks before it advises.
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