Mail digest function

Can anybody tell me how a mail digest works?
I basically know that I will receive one mail including smaller bundled messages instead of every single message. But I am wondering how this is done technically. Where are the messages stored before they are sent to me or the person who wants to receive the digest? Do they reside in a special folder? How is it done that the mail server knows who wants to receive a digest and who wants to receive the single messages?
Is there a number of messages defined up to which all messages are collected and when reached, the digest is delivered?
I'd like to thank you in advance for your answers!