energy - Where does mass come from in pair production?

In pair production, two gamma rays with > .511Mev can come together to create a positron and an electron.

So two electromagnetic waves E and B fields, with No mass and No gravity and traveling at the speed of light, can create two objects with Mass, with Gravity and with opposite E fields, and with NO B field.

Is there any theories on how E and B fields go about creating mass? It seems the resultant objects both have opposite E fields, but the B field has vanished...where did it go?

In slow motion, as this happens, what exactly goes on to create mass from EM fields?

Can we use EM fields to manipulate this process further? i.e. change the rate of pair production? Or to manipulate the G field of the resultant objects?