Where does the extra mass of a $W$ boson come from in particle decay? I’ve seen everywhere explaining that it can exist because of time/energy uncertainty. I get this. I understand that’s WHY it exists but I’ve still never gotten WHERE the extra mass comes from. Is the mass not actually measured/detected? Nobody seems to be able to explain how we get from mass energy value X to a higher mass energy value Y. Only that we CAN because of uncertainty. I’m probably slightly misinterpreting some concepts but it is not satisfactory AT ALL to just say well it has a higher mass because of uncertainty...
 A: There is no extra mass  going to virtual particles because they are not on mass shell. Read on.
In this link  a virtual photon is defined,second page, needed in calculating the crossection of electron electron scattering.(for low energies)

The internal virtual lines are just a mnemonic in the integral that will give measurable quantities, a mnemonic with the particle name, because it carries the quantum numbers of a photon, BUT the four vector ( energy momentum one) that describes it within the integral has a continuously varying mass, it is not on mass shell.
In addition within the integral, the mathematical propagator that describes the line has the mass of the named particle,  

and here, further down in the first link, is the weak interaction mediated by W 

In the case of the W, which has an enormous mass compared with the neutron
the four vectors are way off mass shell within the energy constraints of the interaction.
If you study  further into this, $e^+e^-$ scattering for example, one can see scanning from low to high energies the width of what are virtual particles by the increase and decrease in the cross section. (in this linked plot one could use the Heisenberg uncertainty for energy time, to estimate the lifetime, as long as measurement errors have been taken into account)
