Data is an outgrowth of the last full copydown he did into B4 in 2379.
This Data personality and memory set was uploaded into the Daystrom Institute mainframe along with Lore, Lal and B4.
A second Data personality was taken by Alton Soong and continued to develop in a quantum simulation as we saw in Season 1. This Data personality was the one Picard interacted with in the interim between his biological death and the conciousness transferral to syntheic form. This Data personality asked for Picard to terminate his existence.
The Season 3 Data is the combination of the *personalities* of Lore and Data and the memory files of Lal, Alton Soong and B4. During the "mindscape battle" Data essentially took over Lore form the inside out. Lore has ceased to exist as a unique entity and has been fully integrated into Data, with Data being the dominant and majority personality and memory entity.
Data is Data. Data is not Lore, this is not Lore with Datas memories. Lore is dead. Lore is gone and Lore has ceased to exist.
There is only Data.