A key point to understand is that while emission of a W- boson by a down quark that transforms the down quark into an up quark, which W- decays into an electron and an electron antineutrino with or without a photon, is the "usual" form that beta decay takes when no other decay paths are possible due to a lack of energy-momentum in the system, if the emitting down quark is sufficiently energetic, other possibilities can and do occur.
For example, the W- could decay instead into a muon and muon antineutrino, or a tau lepton and tau antineutrino, rather than an electron and and electron anti-neutrino.
In general, any possible decay of a W- boson that preserves a number of conserved quantities (e.g. baryon number, lepton number, lepton flavor number, electric charge, angular momentum, and net color charge) is possible and has a predictable probability which is at the first order equal for all possibilities, when the aggregate rest mass of the post-W- boson decay system is less than or equal to the pre-emission mass-energy of the system, and the equations of the weak force spell out precisely how likely each possibility is and how rapidly the decay will take place, etc. This is possible even if an intermediate state in the decay chain is seemingly prohibited by mass-energy conservation in which case the particles in the intermediate states are called "virtual particles."
The weak hypercharge of a particle, in turn, determines its probability of emitting or absorbing a W boson.
The same general principles also apply to the emission and absorption of Z bosons.
In general, a particle is fundamental in the Standard Model when its properties cannot be determined from first principles from the Standard Model equations and other particle properties (although in some cases the number of available degrees of freedom can be localized in different ways - for example, you can determine anyone of the parameters (1) Weinberg angle, (2) the W boson mass and (3) the Z boson mass from the other two parameters).
No equation in the Standard Model can tell you, a priori without experimentally measurements of these parameters, the charm quark mass, or the probability that a charm quark that emits a W boson will become a down quark (which it does about 5% of the time) rather than a strange quark (which it does almost 95% of the time). In contrast, it is possible in principle to determine the exact mass of a proton from the equations of the Standard Model and the properties of up and down quarks and gluons and electrons, and this has in fact been done to a precision of about 1% (it turns out that precision measurements of small quantities on a percentage accuracy basis is harder than precision measurements of large quantities like the top quark or tau lepton mass).
Put another way, in the Standard Model, the only deeper kind of stuff is mass-energy and other conserved quantum numbers that are not summed up in a particular kind of particle.
