The original idea from the Matt Jefferies days was that the entire power plant was in the nacelles. All of the original creators were WWII veterans. Roddenberry (and Jefferies, too, I believe) were familiar with bombers. The power plant on a bomber was the engine. The more engines, the more power plants. This was how the Enterprise (and, by extension, all Starfleet and Federation ships) was envisioned. The entire power plant was in the nacelle and the more nacelles, the more power. This was why Franz Joseph designed the destroyers and scouts with only 1 nacelle and the dreadnought with 3 nacelles.
TMP tossed this idea on it's ear when it introduced the warp core. Instead of matter antimatter reactions being in the engine, it took place in the core. Ever since then people have tried to retcon the TOS Enterprise to have a warp core and all the ships must have a warp core. Some fandom has suggested that between TOS and TMP there was a massive leap in technology equal to the leap from steam engine paddleboats to diesel engine ships
TMP tossed this idea on it's ear when it introduced the warp core. Instead of matter antimatter reactions being in the engine, it took place in the core. Ever since then people have tried to retcon the TOS Enterprise to have a warp core and all the ships must have a warp core. Some fandom has suggested that between TOS and TMP there was a massive leap in technology equal to the leap from steam engine paddleboats to diesel engine ships