This part, I don't know, but default magic packet has 6 bytes of 0xff, followed by the mac address sixteen times in a row, so it's a fairly simple state machine as the packet comes in. The AMD whitepaper others linked might have details?

> how, it wakes up the system via PCIe

Pci-e pin 11-B is wake#. PCI 2.2 added PME# on 19A which does the same job for PCI. Pull it high (I think) to wakeup the host. I don't think there's a pin for this on ISA, so you'd need some system specific connector to wakeup from an ISA nic.

> how switches route the frames to the port which has/had the client.

Ethernet switching is a whole different thing. You can send a broadcast frame and those should get flooded to all ports. If you send a unicast frame, the switch looks up the destination mac in its address table, if present, it sends only to the port where that address was seen, otherwise it floods to all ports.

"How to send a magic packet in $LANG" isn't very interesting to me. There are plenty of guides for it, and I remember actually doing it 20+ years ago with a short PHP script.

Even at the time, the task didn't seem like "enough" for a show-the-world blog post. A dramatically shortened version (no validation, error handling, logging, etc.) for your amusement:

    // Given $macAddress and $addr and $port
    $macAddress = str_replace(":","",$macAddress);
    $macAddress = str_replace("-","",$macAddress);

    $header = pack('H12','FFFFFFFFFFFF');
    $payload = pack("H12",$macAddress);
    $packet = $header . str_repeat($payload,16);

    $sock = socket_create(AF_INET, SOCK_DGRAM, SOL_UDP);
    socket_set_option($sock, 1, 6, TRUE);
    socket_sendto($sock, $payload, strlen($payload), 0, $addr, $port);
    socket_close($sock);