Alright let me try to answer:
"if i buy a bivy would i need a tarp? wouldn't a bivy protect you from the elements to a degree."
There are waterproof bivys and then threre are water resistent bivys. A water proof bivy will allow you skip the tarp but you will have no room to cook and sit up because you will need to be zipped up in your bag during a storm. I think most waterproof bivy users are alpinist who need a small shelter most likly to be used in a snow shelter or somthing like that as is the origin of the bivy.
"Also if i buy a tarp do i need a bivy? A tarp protects you from rain, so couldn't i just buy a small mosquito net for the head of the sleeping bag?"
In the begining people used larger tarps with good coverage something like 8x10 was standard tarper size. You didnt need a bivy as the margin around you was large enough to keep splashback and windblown rain away from you and your bag and yes, all you needed was a headnet or just hang some netting from underneath. As people tried to go with smaller sized tarps to reduce weight, reduce the space requierments for camp, and to reduce the amount of surface for wind to catch ( i.e. more storm worthieness) they found the need to protect themselves from splashback and wind blown rain as now there is less of a margin around them. The breathable/water resistent bivy was the answer. It protects from accational rain blown under the tarp, it adds warmth, it adds bug protection, it has as small of a footprint as can be had on the ground, and the small tarp gives room to cook, sit up a bit and do other things during a storm while in your bivy. It also allows you to skip the tarp set-up if you are confident it will not rain during the night making camp set-up and break-down a breeze. I have tried a lot of different tarp-tents and tents and I still preffer the bivy/tarp setup over all others.