How would you make an FE audio visualizer?

I have searched the internet for a while but none were helpful to me, so I would like to know if you guys knew how they are made so I could construct my own.

You would have to Invoke the client to get the loudness. You can use RemoteFunctions to do this. Server Script:

local LoudnessRemote = Instance.new("RemoteFunction")
LoudnessRemote.Name = "LoudnessRemote"
LoudnessRemote.Parent = game:GetService("ReplicatedStorage")`
function GetLoudness(Player,Audio)
	return LoudnessRemote:InvokeClient(Player,Audio)
end
print(GetLoudness(game.Players.Player1,workspace.Sound))

Local Script:
local LoudnessRemote = game:GetService("ReplicatedStorage"):WaitForChild("LoudnessRemote")
LoudnessRemote.OnClientEvent = function(Audio)
	return Audio.PlaybackLoudness
end