When USPS tracking says delivered, in/at mailbox it means that the package was delivered into your mailbox or on your porch or near your house.
But most times when you see delivered, in/at mailbox the package should be in your mailbox if you have a curbside mailbox.
So if your USPS tracking says delivered, in/at mailbox go out to your mailbox and check your mailbox for the package and if it's not there then look around your porch or somewhere that the USPS mail carrier may have hidden the package to keep it out of sight.
If you still cannot find it then contact your local post office with the USPS tracking number as sometimes a package may get delivered to the wrong address.
If you get your mail from the post office then it would say delivered to P.O. Box and a key will usually be in your mailbox.
Or if you get mail in a cluster mailbox then the mail carrier will put a key in your mailbox and put the package in a parcel locker on the cluster mailbox.