Hi,
IMHO: Don´t waste your time with circuits from such unreliable sources.
Maybe - if you omit the short circuit at the output - you may hear something. But don´t expect any really useful.
To your questions:
* there is a brief - not 100% correct description on their website.
* read the LM386 datasheet (it´s a very old IC of bad quality. Nothing one would use for serious circuits)
***
There are tons of good information
* how FM transmitter work
* how FM receivers work
* of simple, good discrete FM reciever circuits (with description)
* of simple, good receiver ICs, with datasheets, application notes, design notes
Good, reliable informations come from semiconductor manufacturers, universities, engineers with good reputation.
Klaus