Source, Medium, Channel & Default Channel Grouping

Source and medium describe where a visit came from (like "google / organic" or "facebook / paid social"). GA4 rolls those up into broader channel groupings — Organic Search, Paid Social, Direct, and so on — using its default channel grouping rules. Crucially, GA4 offers both a session-level version (how did this visit arrive) and a first-user version (how did this person originally arrive, even if this visit was different).

Picking the wrong one of those two changes the story your report tells. If someone found you through a Facebook ad months ago and comes back today by typing your name into Google, session-based source says "organic search" while first-user source says "paid social" — and a report built on the wrong one can make a channel look far more or less valuable than it is.

Which channels are actually driving my traffic?

Ask datanephew for a channel breakdown — it can tell you whether you want the session view (how visits arrived today) or the first-user view (how people first discovered you).

Is this traffic new or people coming back through a different channel?

Ask datanephew to compare session source against first-user source for a given channel — that distinction is exactly what separates those two questions.