If you are interested in how a 5G EN-DC Option 3 connection is set-up between the mobile device and different components in the network the best but also the hardest way is to look up the procedures in a number of different 3GPP specifications. 3GPP TS 37.340 is a good starting point for EN-DC. Another interesting source that I have just discovered is over at EventHelix.com. On the page, there are a number of links to call flow diagrams that aggregate a lot of information from different 3GPP specs and to a blog post with some more details and a video at the end. Interesting stuff!