I am new to c++ and coding in general. so this question might be noobish. what is the difference in using type int64 or int64_t. I saw that one of the software devs modified their source on github and all of the int64 to int64_t.
int64_t is a Standard C++ type for a signed integer of exactly 64 bits. int64 is not a standard type.
The first C++ standard didn't have fixed-width types. Before int64_t was added to Standard C++, the different compilers all implemented a 64-bit type but they used their own names for it (e.g. long long, __int64, etc.)
A likely series of events is that this project originally would typedef int64 to the 64-bit type for each compiler it was supported on. But once compilers all started to support Standard C++ better, or once the person who wrote the code found out about int64_t, the code was switched over to use the standard name.