These products are not certified, they merely have been verified and issued a Declaration of Conformity. The WiFi module on the WiFi shield itself has been certified, as indicated by the FCC ID on its label.
To explain what that means, I am going to give a simplified explanation. If you are looking to make a commercial product and you are not sure what you need to do to be compliant, hire out a professional to help you through the process.
In the eyes of the FCC, devices fall into one of two categories, devices that are intentional transmitters, and devices that are not. Devices that are intentional transmitters need to be certified. Certification is a process of making sure that it is only transmitting on the frequency band it specified, that it is maintaining certain power limits, and that it will not cause interference with other devices. This is a very expensive testing procedure.
Devices that are not intentional transmitters, only need to get a verification. What this means is that the devices is not designed to transmit anything and is test to makes sure that it indeed does not. If you pass the verification you are issued a Declaration of Conformity basically stating that the device is not unintentionally transmitting any frequencies. This is usually a lot less expensive than than certification (but still in the few thousand USD range).
So taking the WiFi shield as an example, the module shown above circled in red is an intentional transmitter, i.e. it is designed to transmit on a particular frequency band (in this case 2.4 GHz ISM band). The device is certified to make sure that its emissions are within FCC specifications and it is issued an FCC ID.
Now at this point only the module is certified and is good to be used in a product, however, now that it is on a shield with extra circuitry added to it, the extra circuitry could be unintentionally transmitting. So now the whole shield is tested to make sure that the added circuitry does not have any emissions exceeding the FCC specifications and is issued a Declaration of Conformity.
So now fast forward to making your own thing, the module is certified, the WiFi shield and the Uno are verified, you add your own additional circuity around those components that may emit electromagnetic fields. You now may have to get your product verified to ensure that the added components to the system have not caused it to unintentionally emit radio frequencies.
The whole process, conditions, and requirements are a lot more nuanced than my description above. So this should in no way be used as the basis for product development or legal advice.
(Image borrowed from https://www.arduino.cc/en/Main/ArduinoWiFiShield101)