At the lowest level, the instructions move data between memory and a small number of registers. Arithmetic can be performed on the registers and the result written back into memory.
Let’s say you want to get z=x + y.
What you have might look something like this:
Copy the contents of memory location 100 into register 5.
Copy the contents of memory location 120 to register 7.
Add the contents of register 5 and register 7 and put the sum into register 5.
Place the contents of register 5 into memory location 130.