Wastegate regulates how much boost the turbo makes, BOV regulates backpressure to the turbo.
Internal wastegate is attached to the exhaust housing and uses an armature to operate a paddle. External uses a spring/diaphragm to allow exhaust gas to flow past the turbo housing.
Once the throttle plate closes on a carb/throttle body, boost has nowhere to go but back towards the turbo. This can cause damage to turbine wheel/housing and kill the turbo. BOV helps vent that boost to atmosphere or back to the turbocharger.
