Nier-Visualizer | A lightweight and efficient Android visual library | Android library

This is the goal that I am trying to achieve:

I would like to be able to take the audio recorded from the phones microphone and process the levels at given frequencies into an array that I could use to create a sort of "Bar graph" visualizer. I also need to calculate the BPM of the song playing to know the cadence at which to update the visualizer at.

What I am really looking for is just getting the frequency array and BPM calculations. I can deal with the actual visualization part if I can just figure out how to process the audio.

This is pretty much what I'm looking to make:

From the research I have done, it looks like a possible solution is using FFT (Fast Fourier Transform). For that I found this: stackoverflow. But that stackoverflow thread is for Java, and my app is currently written in Kotlin. I have read that it is possible to import Java into Kotlin, but I haven't been able to make it work in the couple of attempts I made. So maybe some advice on that would help.

Anyways though, I did end up finding a library that was written for Kotlin Nier Vizualizer. It does have a vizualization similar to one I'd like to reproduce, but for the life of me I cannot figure out where I would pull the frequency array from. I tried reading the buffers before they were passed into the visualizer, but it's just an insanely long string that I'm recieving. I mean, I'm not sure what I was expecting, but I could definitely use some help understanding how I would translate that into a data format that I can actually use.

Here's an example of where I'm at: KeyFrameMaker.kt