If a domain expires and is neither renewed nor restored during Redemption Grace Period, then it enters a stage called Pending Delete. During the Pending Delete period, the domain can no longer be restored, renewed, or recovered by the previous owner or registrar and the domain is scheduled to become available for registration in 5-10 calendar days. Once the 5 days have passed, the domain is be released and the zone file are deleted.
When the Pending Delete period ends and the domain is released, it will be available across all registrars and can be picked up by Backorder systems or registered.
You can read more about the life cycle of domains and the Pending Delete period on ICANN's website.