Multiply Accumulate (MAC) unit easily explained

Been looking for a good explanation on MAC operations but I found nothing that satisfies my curiosity.

I get the point that in DSP processing MAC units are required but that is about it.

I was looking for a lower level explanation of the MAC unit and operations. I kept on finding documents/white papers on google about FIR filters and FFT?!

So in order to place the MAC unit and operations in perspective:

How does the MAC unit tie in with FFT? If at all.

Whats going into my MAC unit? What is coming out of my MAC unit in a FFT application?

Whats going into my MAC unit? What is coming out of my MAC unit in a FIR filter application?

In general does the MAC unit have a limit that it reaches before it cannot do any more processing?

Is the MAC unit synonymous to the Arithmetic Logic Unit (ALU)?

A MAC operation is simply the sequence of two elementary operations:
- two operands B and C are multiplied
- the result is added to the accumulator
A = A + B*C
That's it.
It is the basic operation used for FIR filtering, that's why you find a dedicated unit in DSPs.