Does "lowering the gaze" mean not looking at all? And is looking at passersby from a vehicle window, or a quick, unintentional glance, or looking at a doctor while he is explaining in order to understand the lesson, considered forbidden?

Lowering the gaze is obligatory concerning what is unlawful. A sudden glance is pardoned because it is unintentional, based on the Prophet's (peace and blessings be upon him) saying: "O Ali, do not follow up one glance with another, for you are allowed the first, but not the second." And also based on his saying: "Avert your gaze." As for a woman intentionally looking at a non-mahram man without desire or fear of temptation, some scholars have deemed it permissible.