Where is the 128-bit result stored from a multiplication of two 64-bit Integers on a 64-bit computer?
Would it be correct to assume that two parallel 64-bit Integer multiplications producing a 128-bit result could be performed by AVX-512? Two 64-bit multiplications plus four Registers for the two 128-bit results?
How is the result stored in a C++ program? Is it two 64-bit Integers for the 128-bit result?
Are these two 64-bit Integers as the 128-bit result known as Limbs?

This is really a question of what can be achieved. Ideally I would like to do Arbitrary Precision Arithmetic on millions of digits.