Except for how to construct a personal APRS terminal, there is little
straightforward explanation how the basic APRS network does its stuff.
Its like pulling teeth.

AFAIK, here is a simple outline:

[GPS->]TNC->Radio->Tx --------------------|
                 |                        |
Digipeater <-> Digipeater <-> Digipeater  |
                 |                        |
               IGATE(s) <-----------------|
                 |
None-Core internet server(s)
                 |
Core internet server(s)
                 |
Internet archiver (like findu.com)

You make up a APRS terminal [GPS->]TNC->Radio->Tx to transmit your
position (or even weather data) - the GPS is optional for tracking
your moving position.

Your position is transmitted to a digipeater and then to another
digipeater etc. until it reaches an IGATE station

An IGATE station (some 60 in US alone) sends your data through
internet to "non-core internet servers" (about 30? in the world),
which send it to "core servers" (about four) which make their data
available to APRS archivers like findu.com.

Note: if you are near an IGATE your data signal does not have to be
bounced around different digipeaters but goes directly to the
internet. If you are in remote area without digipeater or IGATE
nearby, you could do yourself and others a favor by setting up a IGATE
(you need an antenna, radio, TNC, and a computer connected to live
internet) and receiving APRS terminal signals directly to the
internet. Legally you don't even need an amateur ham license to
operate a receive-only IGATE which only sends the data it receives to
internet APRS servers.

There is more complexity to APRS system through satellites -
used by ships, etc, to report their position worldwide.