Maybe it's different in US, but in UK all deliveries from Amazon are done by 3rd party companies - DPD, Hermes, Parcelforce, etc, even if the tracking website says "Amazon Logistics" - it's just a front company.
In the US, they are all individual "contractors". They call them Flex Drivers. They are similar to Uber drivers. Individually hired but are technically contractors. So Amazon does background checks on them but technically they are not Amazon employees, they are "self employed". Which is why many of them drive things like Uhaul vans for deliveries. They try to cram as many orders as they can into a van, and rush them through, to get the biggest possible payout.