I remember the extended version of that song well. I used to have it on vinyl many moons ago. One of the best late night listening jams ever.
A lot of the magic you hear in there is plain old top notch editing and mixing by the Motown engineers and producers. The reverb effects weren’t hard to achieve in the right sized room. It could even be simulated with tape delays and such.
Norman Whitfield and Barrett Strong were the chief producers of Motown artists in that day. They were beginning to stretch out and experiment with the standard top forty format of the day. Lots of experimental things were going on in the studios, and creativity was suddenly turned up to 11 (as we used to say).
That was before Motown moved out of Detroit, right? In any event, after the relocation to the West Coast, the distinctive sound they had was largely lost.
One from around that same era with great instrumentation was It Takes Two - Marvin Gaye and Kim Weston, with a fine instrumental bridge.
Overdubbing has been using in commercial music recordings since the 1920. Les Paul helped to bring the tape version of overdubbing to the forefront in the 1950s. He also showcased tape delay reverb, varispeed/flanging and phasing. All this stuff was old hat by the 1970s. It’s unlikely that any unusual techniques were used to record in the song in question.