How can a 64-bit processor address 2^64 different memory locations at a time? I understand that a 64-bit processor can hold a 64-bit long address and that a 64-bit address can represent $2^{64}$ different values. But I don't understand why that processor can address $2^{64}$ different locations in memory at one time and thus process billions of billions of GB at a time (theoretically). 

I haven't found a satisfying answer on google. On google they just explain that a 64-bit address can represent $2^{64}$ different values and thus it equates to the processor being able to process $2^{64}$ values (or memory locations) at a single time, though I am interested in as much detail as possible regarding how exactly that looks like. 

64-bit address means for me that this address can have one of $2^{64}$ possible values and not that it can process $2^{64}$ at a time.
It's not clear where you found this claim so it's hard to tell exactly what it's supposed to mean.

But, obviously, no, the CPU can't access every memory location simultaneously. Each CPU instruction can only access some fixed small number of memory locations at a time, and only a fixed small number of instructions can be executing simultaneously.