On most cameras, Bulb is essentially a Manual-mode shutter setting. The camera does not automatically determine exposure because it cannot know beforehand whether you will hold the shutter open for 5 seconds or 30 seconds.

In normal Bulb use:

• shutter time: controlled by you
• aperture: set by you
• iso: set by you

So Bulb is usually the most manual exposure mode available.

If a camera happens to allow Bulb in shutter-priority or another automatic mode, it generally cannot meter true Bulb duration in advance. In those cases, behavior is camera-dependent: some bodies may fall back to default assumptions, and some effectively treat it like a fixed long exposure such as 30 seconds. Auto ISO behavior can also vary, but it is often limited to a default/base sensitivity.

So the practical answer is: in Bulb mode, you should expect to set aperture and ISO yourself, and not rely on automatic exposure to calculate them from an unknown shutter duration.