Delivered in/at mailbox with USPS tracking means that your package or mail was delivered inside and to your mailbox.
If you get curbside mail at a curbside mailbox, the delivered in/at mailbox with USPS means that the mail carrier has placed your package inside your mailbox.
So you should check your mailbox to see if the package is inside of it.
If you get your mail at your post office, then your package is either inside your post office mailbox or they have left a notice to pick it up.
Or if the package was too big for your mailbox at the post office they may have placed a key inside your mailbox that goes to a parcel locker inside the post office.
In that case, you take the key to the parcel locker with the number, insert the key and unlock it and retrieve your package.
The key then stays inside the parcel locker lock.
If you get mail at your curbside mailbox and the package was not able to be left in your mailbox or at your house, the mail carrier should've left a notice to pick it up at the post office.