Trying to understand how this electronic wizardry works ...

BMW designs the car to have a bulb with two filaments but only two conductors going to it?

And the bulb (made by Philips, Sylvania, or ...) has two filaments and four conductors, so that non-wizards can still use them?

So BMW has a connector that splits its two conductors coming from the magic box into contact points to access all 4 conductors going into the regular 2-filament bulb?

I don't have any interest in LEDs, but like to understand how things work, even the magic stuff, if I can.

Checking my own notes from when I replaced the right front turn signal bulb on my 2001:
Spec is Amber 5W/21W (same thing for the rear turn signal lights), which is what I put in (Philips brand). Removed bulb was clear, not amber, but both filaments were intact, and measured finite resistance. Rear, side, and instrument panel had been flashing rapidly, with front not lighting up. After replacement, worked fine.

I wrote that after fixing it, and it did not make sense at the time (and still not completely now) - I expected at least one of the filaments to read infinite resistance, since there was a problem. So the bulb must have had 4 contact points or I would have noticed that; but I did not notice the number of wires coming to the connector.

So could it be that BMW uses both filaments in parallel (same voltage across both filaments at all times), pulse width modulating the 12VDC to get the desired power? Then maybe it monitors the combined resistance, and even if both filaments are intact and conducting, if the resistance moves out of spec, it acts as if the bulb has failed? Maybe???

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2001 X5 3.0i, 203k miles, AT, owned since 2014