How is channel frequency response/channel state information related to free space path loss?
In the OFDM system, such as Wi-Fi, there are infinite paths exist, assume it is in free space, and only one path exist,

Exactly. So if you use OFDM in a scenario where FSPL describes your transmission sufficiently, then you're doing something wrong, because you don't need OFDM, as you have a flat channel.
We can still describe large scale fading in multipath channels using models similar to your FSPL formula, but neither of your equations hold! Then, we're describing distance-dependent stochastic properties of the random variables that make up your CSI.
You need to cross over from your deterministic &quot;this channel has a defined attenuation&quot; model to a stochastic &quot;and here's how we describe the properties of random variable, whose realizations are channels&quot;.