Sorry, was on vacation for a bit. Coming back to it...

So the IP address is in the layer 3 header. Sort of, but not really. IP is a L3 technology, and when IP is used to perform the Layer 3 tasks, an IP Header is added to each packet. Within each IP header is a bunch of information, but most significant (to this discussion, at least) is a Source IP Address and a Destination IP Address.

So the L3 header, is the IP Header, and contains a Source IP address and Destination IP Address

> arp with mac is only used to send to my outgoing gateway. Right? Yes. Unless you are speaking to something on your own local network, in which case ARP will be for the final destination of the communication (aka, the device with the IP address in the Destination IP of the IP Header)

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The Article did not go into the details of the ARP Cache, because it differs for each device. Hosts cache ARP entries differently than Routers, which cache ARP entries different than Firewalls, etc. It ends up being dependent on the vendor. But typically, entries in the ARP cache are flushed based upon a standard timer. Anywhere from 30 seconds to 4 hours.

The other thing that updates ARP is new ARP Requests. If I ARP for my gateway, I keep that for (say) 4 hours. If my Gateway at some point ARPs for me, that causes my own ARP entry to also be updated and refreshed. Alternatively, if my Gateway issues a gratuitous ARP, that will also update my ARP Cache.

A Gratuitous ARP is essentially an ARP Response send to the whole network which was not prompted by an ARP Request.

@JDlugosz Answers above. Let me know if you need any more help.