The 32-bit operating system is designed for 32-bit CPUs. The 64-bit operating system is designed for 64-bit CPUs.
A 64-bit CPU can run both 64 and 32-bit operating systems, while a 32-bit CPU can only run a 32-bit operating system.
A 32-bit operating system can process 32 bits, that is, 4 bytes of data at a time; and 64 bits can process 64 bits, that is, 8 bytes of data at a time.
Theoretically: 32-bit = 2^32B = 4 * 2^30B = 4GB, which is the upper limit of the memory for the 32-bit ordering process.
In the 64-bit architecture, there are 46 address lines, so the largest physical address is 2^46B, which is equivalent to 64TB, and the available address space is so large (so far).