You’re about right with the difference between Warranty and SmartNet (or Meraki Now as I believe it’s called on the Meraki kit - or was initially).
The Warranty covers you for hardware replacement (in theory only so long as the device has a corresponding license), most devices are lifetime warranty, although some like camera and outdoor APs aren’t. With the Warranty you get advance replacement of a failed device, with a next business day replacement within commercially reasonable efforts. Now depending where you are in the world, and where the closest devices are that may or may not end up being next business day.
SmartNet/Meraki Now improves on the Warranty by allowing you to extend the hardware replacement period for those devices that aren’t covered for their lifetime, but again you need to have a valid license for the device. It also allows you to purchase improved replacement times, e.g. 24x7, 24x2, and with or without an engineer. There is also a next business day option, which almost guarantees what it says, so better than the Warranty. The difference is that with SmartNet/Meraki Now Cisco actually make sure they have spares located close to the install address.
