How is it that 1GB (1,073,741,824) is worth 1 billion btyes roughly yet it is also within the 4 byte range where 32bits = 4bytes?

Question

How is it so that 1GB (1,073,741,824) is worth 1 billion btyes roughly yet it is also within the 4 byte range where 32bits = 4bytes & 8bits =1byte with 1,073,741,824 being a