It seems I fail to understand few things about DH's functionality in the context of IKEv1. A little bit of context first:
During IKE Main Phase, with the 3rd and 4th messages, the peers each exchange Nonces and key exchange data. I understand that each peer takes both nonces and combines them with the PSK to obtain the SKEYID, thus both obtaining the same SKEYID.
First question: What exactly is this "key exchange data" and what is it used for?
Second question: What is the process if there is no PSK configured? What if I use X.509 certificates? What do the peers use instead of the PSK?
Then I understand that the peers each derive 3 more keys used for different purposes, from the SKEYID.
Third question: How are the 3 keys derived from the SKEYID? What is the SKEYID combined with or what is the process?