@Jessica Smith one thing I didn't realize when I recorded the video: the Primary channel group setting is not just saying, "I want this channel group to be my primary channel group", it is its own channel group. So for example, if you create a custom channel group with AI as a channel on November 8th, but don't set it as your Primary channel group right away, then decide to set it as your Primary channel group on January 1st, the Primary channel group will only start assigning traffic to the AI channel on January 1st.
Learning this, I'm on the fence about how I think about the primary channel group. On the one hand, it makes sense to change the definition the minute you decide that certain source/medium/campaign values should be assigned to a different channel so you start collecting data. But on the other hand doing so means that the primary channel group will become an increasingly opaque mixture of channel assignments over time.