Short: In most cases "BULB" is a speed setting accessible only in MANUAL mode so you will have full control of aperture and ISO settings.
"Bulb" mode is the ultimate manual mode.
Bulb is accessible in Manual mode and MAY be accessible in Shutter-speed priority mode.
It COULD have a setting of its own but is usually at the low end of the shutter speed range after the slowest speed that is settable.
In Manual mode the exposure is not metered so the camera does not need to "know" what the exposure is. ISO will be what you set it to. If you have an auto-ISO feature the camera maker may decide to make it track in some way to try and adjust for likely exposure but, as the camera cannot read your mind, setting it to the best quality ISO setting within the auto range probably makes most sense.
It is unlikely that you can, BUT IF you can access Bulb setting in Shutter priority mode (on eg my Sony A77 you can't) as you move down through speeds: 1/8, 1/4. 1/2, 1, 2, 4, 8, ... the aperture will increase to correct the exposure. If you have an auto-ISO feature the ISO may also adjust. If the speed/aperture/ISO settings "top out" they will remain at maximum value. When you reach bulb mode "all bets are off", the camera again cannot read your mind and each manufacturer will have made their own decision.