People on the paid Team and Enterprise plans can now sign up for a beta to get access to new and powerful macOS runners for x64. Access is requested via the beta sign-up page. Once your request has been approved, an email will be sent with additional details. The new XL runner option provides developers with 12 cores to execute their Actions workflows on and improve build times.
To learn more visit the docs. Information on pricing for the new macOS XL runner is here.