Ecommerce tracking allows you to measure the number of transactions and revenue that your website generates. On a typical ecommerce site, once a user clicks the "purchase" button in the browser, the user's purchase information is sent to the web server, which carries out the transaction. If successful, the server redirects the user to a "Thank You" or receipt page with transaction details and a receipt of the purchase. You can use the analytics.js library to send the ecommerce data from the "Thank You" page to Google Analytics.
GA4: Measure ecommerce
Google: Measure ecommerce in GA4
Migrate ecommerce data collection from UA to GA4