PC build noob help

-edit- Sorry for that massive text wall, I posted that really late and kinda tired so wasn’t thinking straight. I won’t do that again.

So yeah I wanna build a PC and was looking at some CPUs to start and had some questions about certain terms for the specs.

I was looking at a comparison chart and the AMD 2700X has its boost frequency listed at 4.3GHz. (I assume boost frequency is what it runs at while overclocking) But the Intel 9700K had 4 seperate boost frequencies for 1 core, 2 cores, 4 cores, and 8 cores. They both have 8 cores so why does one only have one speed and the other 4 separate ones?

The 2700X also had two different memory speeds listed for dual rank and single rank. Does that just mean you need different speed memory cards (cards/chips/?) depending on if you have one slot or two filled?