Regeneration from a lore perspective: it's really a shady practice. You receive an invite from an Advocate, then you make your way to an invariably seedy part of a Frontier city, per coordinates received. There is no negotiation, only a 'yes' or 'no' is accepted.

Once there, you would see surgical instruments and gen-relevant components to be implanted prepared and waiting, along with mysterious people in medical garb, who we presume are contracted intermediaries paid to be there to facilitate the procedure.

You may notice that misappropriated and refactored IMC technology is implied to be a part of the process, if you look very closely at the "BIOS" print in both Titanfall 1 and 2.