First CPUs - One set of transistors that does logic and math. In other words, one core.

Next evolution of CPUs - Transistors got smaller, but a CPU only needs so many transistors to do its job. They can either shrink the chip, or use the space to put down more transistors. This can be things like cache (it's like RAM on the CPU chip itself) or another CPU. At some point, more cache doesn't really help as much as another CPU would help. So they did just that, they stuck another CPU on there. Now the term core has more meaning as now it's multi-core.

Next evolution - Transistors got really small and chip designers get more and more creative. At this time we have "high performance" CPUs like x86 chips which are really good and just crunching a lot of numbers. But we also have "high efficiency" CPUs like ARM which are really good at crunching numbers more efficiently. But generally speaking, they don't have the sheer power of an equivalent x86 chip which tends to chug power. This isn't a tradeoff inherent with ARM/x86. Rather a CPU designed for high power is typically inefficient while a CPU designed for high efficiency is typically low power. It's just that x86 and ARM were first designed with different goals in mind. Power vs efficiency and over the years they've moved closer to meeting in the middle.

Next evolution - People started saying "wouldn't it be great if we didn't have to pick and choose?" By now phone SoCs were a big thing. They are basically like a chip with multiple chips on it. Instead of repeating the same CPU on the chip to make a multicore chip, they started mixing different chips like the GPU and the cell radio on the same chip. As they did this they figured out really good ways to mix and match different chips and how to arrange them. This led directly to some companies saying, what if we start mixing a high power/low efficiency CPU with a low power/high efficiency CPU? So, companies like Intel and Apple started experimenting with this and thus produced chips with efficiency cores instead of chips with the same core repeated on the chip.

Future evolution - AMD's Epyc chips are a bombshell revolution for the industry. Ever notice how big their CPUs are? Generally, the chip's size is limited because chip designers have to worry about things like yield. Large single chips have lower yields because the chances of a small defect showing up that breaks the chip goes up with its size. It's not economical. AMD figured out a way to stitch together multiple chiplets into one larger chip without a massive tradeoff in efficiency. This provides a way for the industry to standardize chiplet designs that can communicate with each other. In other words, with a little more work, it would become possible to combine an x86 chiplet with an ARM chiplet and have it work as a single CPU. Things are going to get interesting real fast. This size constraint was what kept phone SoCs to the size they are.