Here is how to know which marketing channel is working, in three steps. One: put UTM parameters on every link you control, so traffic arrives labeled instead of anonymous. Two: read the Traffic acquisition report in GA4 (Reports, then Acquisition, then Traffic acquisition) and sort by your money event, not by sessions. Three: add a required free-text "How did you hear about us?" field to your checkout or signup, because customers will tell you things no tracker can see. Where the analytics and the answers agree, spend more. That is the whole system.
You do not need an attribution platform or a data warehouse. Those exist for companies spending millions across forty channels. You are answering one question: of the handful of things I am doing, which ones bring people who pay? That is answerable with free tools you already have, in about an afternoon.
This guide walks through each step properly: what UTM parameters actually are and the naming conventions that keep your data readable, which GA4 report answers "where do customers come from" (there are two that look identical and answer different questions), why last-click measurement quietly lies to you and why that is acceptable at your size, why the numbers from Meta, GA4 and Stripe will never match and how much divergence is normal, and how to see the new stuff: visitors arriving from ChatGPT, and GA4's built-in AI analyst that now answers plain-English questions. Written for a founder doing their own marketing, probably at 11pm, with real money on the line.