Helpful rescources for learning and understanding Euler Angles?

I've been looking into the fundemental properties of Euler angles, and found this helpful article. The article provided a handful of useful equations and functions, though it lacked any explanation. For example, it describes a situation where only one axis (from 0 to 360 deg.) is needed, and provides the method/solution:

local direction = part1.CFrame.lookVector
local heading = math.atan2(direction.x, direction.z)
heading = math.deg(heading)

Though no explanation was provided in any way that described the math behind it.

Reference notes were given at the end of the article, which was somewhat helpful, as follows:

local sx, sy, sz, m00, m01, m02, m10, m11, m12, m20, m21, m22 = matrix:components()
--            m12 = ?cos Y sin X
--            m22 = cos X cos Y
--  => -m12 / m22 = sin X / cos X ]]
local X = math.atan2(-m12, m22)

--  m02 = sin Y
local Y = math.asin(m02)

--            m01 = ?cos Y sin Z
--            m00 = cos Y cos Z
--  => -m01 / m00 = sin X / cos X ]]
local Z = math.atan2(-m01, m00)

Though again, there was no explanation given. Any links to videos, articles, etc. that explain euler angles, even if it doesn't involve Roblox CFrames, would be great. Thanks in advance!