It has been posted may times, and using the Search feature will reveal a wealth of knowledge.

In general:

Full battery voltage is applied through a relay to each GP. (Some older models had series-wired GPs, where the full battery voltage applied across a series string.)

Glow time duration is determined by the relay sensing engine coolant temperature. The GPs are on longer when the coolant temperature is lower.

The relay may continue to apply power to the GPs after the starter is released, again dependent on engine model. I don't know which models apply.

As you have gathered by now, it's all controlled by a Glow Plug Relay, which is an electromagetic device with an operating coil and one contact per GP. When the coil is energized by the electronics, the contacts close and apply battery voltage to the GP.

There are electronics that sense when the key has been turned to Pos II, closing the contacts and lighting the GP light. Electronics then determine when enough time has passed to turn off the GP light, and how much longer the relay is to be energized.

If the GPs stay on after the starter has been engaged, they stay at full battery voltage, since the GP relay is just a switch.

A crude, but cheap way to check is to observe the dome light. When you turn the key to Pos II, the current drawn by the GPs should cause the light to dim. The GP light goes out after the minimum time required, indicating OK to Start, but the GPs are still energized. If you don't start the engine, you may see the dome light brighten again when the GP relay releases.

A voltmeter can also tell you a lot.

Hope this helps, and I'm sure others will add information I have omitted.

Best Regards,
Jim