Going back to this-
(Thanks to this thread, I understood now how ambisonic works - it is the same as Mid/Side manipulation of coincident pair of mics, but this is in 3d space and with the possibility of more combinations)
In terms of the fundamentals that's actually a pretty good basis for understanding basic 1st order ambisonics. At it's core ambisonics can be thought of as a further extension of basic Mid/Side.. or more specifically, of sum/difference processing of the basic omni and bidirectional components which in varying combinations form the basis of all 1st order sensitivity patterns.
I don't mean to differ with Len in his reply to that post, and I don't think we actually disagree on that point as I believe was replying with the specifics of implementation in mind rather than the fundamentals.
To clarify, not simply the manipulation of a pair of "microphones" but of three (or four) at minimum. Specifically two crossed figure-8s and one omni for a 2-dimensional single plane 1st order ambisonics (horizontal only), and an additional figure-8 required to extend that to the 3rd dimension (height). Those "microphones" correspond to the 4 channels of ambisonic B-format (W = omni, X = front facing 8, Y = left facing 8, Z= up facing 8 ).
I put "microphones" in quotes above because I'm describing the fundamentals more than actual implementation. Although it can be done using actual microphones of those patterns (doing so is referred to as a "native" B-format recording array), actual ambisonic microphones such those from Core Sound's do not actually use three figure-8s and an omni. Instead they use a different, more advantageous arrangement of microphones which significantly helps improve a number of implementation issues, in combination with some special sauce processing. The actual output of the microphone elements of such an array prior to the application of the "special sauce" is referred to as A-format, and is specific to each microphone design. In typical use, that A format output is what gets recorded to the machine, yet is unusable until the "special sauce" is applied resulting in "universal" B-Format (actual output is either B-format or direct output of some form of mono, stereo or multichannel output converted internally from B-format). Only at that point does it cross from the real-world-implementation-specific realm into the realm of fundamental ambisonic B-format signals which can be be manipulated with sum/difference techniques in the same way as Mid/Side, only with far greater flexibility. More advanced tools and plugins can do stuff beyond simple sum/difference processing, but sum/difference manipulation of the fundamental omni and bi-directional components is the underlying basis of ambisonics.