Not much to add after the great Wikipedia link TheBigA posted. Here's a piece with information from Larry Levine, possibly shortly before his death.
Phil Spector was one of the first producers to realise that a recording studio could be an instrument in itself - and the sound he created over 40 years ago has influenced popular music ever since.
www.soundonsound.com
My view is on the tech side Spector and Levine superbly used room leakage, positioning of the musicians, and Gold Star studio's two live echo chambers to create a memorable sound. From the photo in the link during the prime hit making period Gold Star appears to have two three track machines and at least one mono machine. Sel-Sync may have been an option on the 350 series three track. With a room filled with musicians playing at once they achieved more density (when they wanted it) on one recording pass. Back then live echo chambers were a big deal and gave studios a competitive edge. Capitol Recording Studios has eight echo chambers below the parking lot
btw, Gold Star was located just a few blocks south of the building where I worked. I loved that about Hollywood, there was music, film and TV history everywhere. Did you know Frank Sinatra recorded an album at 5515 Melrose, which later contained KHJ, and then KRTH for a few years?
Also, according to online articles, Abbey Road studio used primarily by the Beatles had two four track Studers and they used half inch tape; and EMI policy was they didn't reuse tape. This and extensive use of Fairchild compressor meant the Beatles printed a fat sound to tape and largely avoided tape noise. Not re-using tape meant one song may have four or five work tapes of a song as it was created by dubbing between the dual four track machines. i.e. fill four tracks on machine A, mix to one track on machine B, fill remaining 3 tracks on machine B, mix to one track on machine A on on fresh tape, repeat as needed. Decades later Giles Martin was able to use the multiple reels to re-mix a given Beatles song on the Love album. He was able to use tracks from earlier reels and got the genuine sound before several tape generations. Four reels would be sixteen tracks of stuff dubbed into to Pro-Tools, to be synced and mixed. Now, instead of four Beatles playing most of the time, Spector had a dozen musicians blended with room and echo chamber sound.