So when Nero's ship modified and how does a Mining ship get Borg Tech?
The entire Romulan ruling council managed to escape before Romulus was destroyed.
Nero, after witnessing the destruction, beams them onboard, extracts information from them and murders them.
One piece of info he gets from the senators is the location and codes for a top secret, advanced military facility which would also be used as a safe haven for the High council in cases of severe threats to the homeworld.
Nero goes there, lies about the senators' fate, gives the codes and is allowed to enter the facility.
Commander of the facility D'Spal shows him some of the weapons they have been working on which are retrofitted Borg technology applied to Romulan designs.
The new tech supposedly gives to a ship superior warp, cloacking and sensor capabilities beyond the Federations wildest dreams.
It also has the ability to learn. It has self-repairing nanotechnology which also anticipates potential threats and modifies systems accordingly, growing the ship to adapt.
D'Spal tells Nero that she will use his vessel as a prototype for the technology and so Nero eventually gets an updated Narada