Tracking snippets live in the Analytics & Tracking card at the bottom of Settings › General in the admin. The card is collapsed by default — click its heading to open it. These are set-once fields, so most programs only visit them at launch. Click Save Settings when you're done.
The Google site identifier — something like UA-12345678-1 or G-XXXXXXX. You'll find it in your Google Analytics account; if you're having trouble, Google's instructions are here, under "Find your Tracking ID and global site tag."
Once it's in place you'll be able to track all of the usual stats you're used to seeing.
Runs on every page. Paste the full snippet, including its script tags. This is where custom tracking goes when a plain Google Analytics identifier isn't enough.
Fires once when a new member completes registration — conversion pixels go here.