I’ve read other questions on here, but I still don’t have the answer that I need. In pair production, where does the mass come from that’s found in the electron-positron pair? I’m extremely novice and recognize that part of my problem with understanding how mass comes from energy is due to the lack of understanding I have for energy’s relation to mass. If someone could help me understand this, I would greatly appreciate it.
The mass does not "come from" anywhere. You should view the produced electrons as excitations of the electron field. The electron field has the property that the energy $E$ and momentum $p$ of an excitation must satisfy the relation $E^2 = p^2 + m^2$, where $m$ is some parameter that we call the mass. Because the photon (or whatever) couples to the electron field, it can produce a pair of these excitations that we identify as electrons/positrons. Given the energy and momenta of the initial particles, the electron and positron are produced with energy and momenta that satisfy the relevant conservation laws and the above relation to the mass parameter of the electron field.
Here is a simple pair production diagram used to calculate crossections to compare with experiment:
Special relativity is necessary to understand it, in addition to quantum mechanics. In special relativity all particles are described by four vectors. The energy momentum four vector has a "length" , a quantity invariant under Lorentz transformations and in the case of a particle it is defined as its invariant mass. It is the mass that describes the electron or the positron in the diagram above.
When two four vectors are added , as in the diagram, the invariant mass is the "length" of their added four vectors. In order for the reaction to happen, the incoming photon has to have the added energy of the two outgoing particles. That is how energy balances in special relativity.
Note the necessity of the off shell gamma at the lower end. A single gamma cannot "decay" into a pair, as its invariant mass is zero and the invariant mass of the pair is at least two electron masses.
An isolated photon cannot decay into an electron + positron because that would disobey conservation of momentum. But a high energy photon in the vicinity of a nucleus can decay into an electron + positron because the nucleus can balance out the momentum.
Rest mass is just a form of energy (and in particle physics it is sometimes called rest energy), and there's no problem with converting other forms of energy to rest mass, or vice versa, as long as you obey all the conservation laws. For pair production or annihilation, that means energy, momentum, electric charge and lepton number must be the same before and after the reaction.