You want files in Site1 that are labeled with Label1 to automatically move to Site2 two years after creation.

In Microsoft Purview Retention Labels, under “Choose what happens after the retention period”, the available options are:

Deactivate retention settings – Ends retention but does not move files.

Start a disposition review – Sends items to reviewers for approval (manual process, not auto-move).

Change the label – Applies a new retention label (but does not move files to another site).

Run a Power Automate flow – Executes an automated workflow such as moving the file to another SharePoint library or site, notifying users, or applying custom business processes.

Since the requirement is automatic movement of files from Site1 to Site2 after 2 years, the only valid choice is Run a Power Automate flow.