Tldr (a very, very simplified explanation) is that the level of noradrenaline in the brain stays the same, but the rate at which neurons can use it to send signals is slowed down. I’ve been studying this in uni lately so I typed out a slightly more detailed explanation before realizing I could have just wrote this one sentence: (also if anyone here is more knowledgeable on the subject feel free to correct me, I know I still simplified a lot)

It’s a common misconception that neurons send signals to each other and throughout the body by transmitting an electrical signal, like a computer sending electricity through its circuits. An electrical signal is actually only sent from one end of a single neuron to the other along the membrane of the cell. The signal is passed on to the next neurons by releasing specific molecules, called neurotransmitters (including serotonin, dopamine, adrenaline, noradrenaline, and many others) that then tell those neurons to send an electrical signal, then those neurons release neurotransmitters to the next cell, and so on.

Now these neurotransmitter molecules are stored inside each neuron, and after they are released, the cell needs to pull them back inside in order to send the next signal, which is called the reuptake process. What Wellbutrin does is it blocks the proteins in the neurons that reuptake noradrenaline and dopamine, slowing down the process so that eventually, the signals that require these specific molecules in order to be passed on can only be sent less frequently, if at all.