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!
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It should be noted that the EventHelix flow is not correct in the last step. Once the EN-DC leg addition is completed there is no RRC Connection Setup procedure over NR. The RRC Connection Setup procedure over NR is only applicable in SA operation.