According to Alan Dean Foster's novelizations, the plans came from scans by a reconnaissance flight sent by the Resistance the moment Finn told them the base location (correction: he told them about its existence, and I think location came from extrapolating the Hosnian system's attack vector).
Easter egg(ish): The flight was led by "Snap" Wexley, who attentive
Star Wars nerds readers will recognize as Nora Wexley's rebellious teenage son from "Aftermath" otherwise largely worthless prequel book.
In the main conference room of the base on D’Qar, an ongoing strategy session had brought together the leaders of the Resistance. Leia, Poe, C-3PO, Han, and an assortment of senior officers including Statura and Ackbar were assembled around a three-dimensional map of an isolated, frozen planet that up until now had not been worth a hopeful visit from a minor trading ship. Finn was present, too, since it was his information about the world in question that had prompted the gathering.
“The scan data from Captain Snap Wexley’s reconnaissance flight confirms everything Finn has told us,” Poe announced to the group.
Wexley spoke up. “They’ve built a new kind of hyperspace weapon within the planet itself. Something that can fire across interstellar distances in the equivalent of real time.” His expression showed his incredulity. “I’ve had my share of technical training, but I can’t even imagine how that’s possible.”