Meta CAPI for Shopify step 5

I followed the guide “Send data to server GTM container”

but I didn’t understand the last step where it says to send data to the server via ga4 or data, how should I do it?

You can find quite a few detailed guides online.
To transfer data to the server, you can use GA4 or Data tag. GA4 is usually recommended for sending data to Google platforms, while Data tag is recommended for all non-Google platforms.
Stape also hosted several webinars covering this topic as well: