There are no virtual things! Every computer / imaginary apple is represented by real electrons on a drive / chemicals in a brain. Even if we are a simulation, we also have to ultimately be represented by something real.

It seems that there are no real things either. As far as we know, every real thing is represented by another real thing. Even electrons are made of quarks, which are probably made of other things we don't know yet.

Addition is abstract phenomena based on math, which itself is abstract, so it can only function in abstract setup.