Where is the "Sync Now" button in Android Studio?

It's the "Sync Project with Gradle Files". And you can press it manually whenever you want.

Generally you want to do it when you change some configs in your Gradle Files like adding dependencies, changed signing configs, change version code, etc.

press Ctrl+Alt+y to Sync Project with Gradle Files

You can use File -> Sync Project with Gradle Files to sync the project manually.

In 2021 the name of the pption has changes to " Reload All from Disk" in the File dropdown menu ( same as ctrl+alt+y )