DDoS ovetloads the capacity of a device to process requests and forces it to miss valid requests. In that sense, it matters not what the target protocol is, assuming you have a high-enough bandwidth as an attacker.
Imagine that your wifi router is set to full block mode on external WAN network. By directing several Gbs of invalid requests at it, the simple router will be focused at contineously dropping the invalid requests, overloading it's CPU capacity. Since requests keep coming, the router will lock-up, causing denial of service to valid users, even having no clearly visible attack surface. This is due to the fact that blocking firewall rule is still a rule, hense it has impact on the device (even if tiny).
The difference between a dumb flood into nothing and an HTTP floor, or Syn flood is that the victim actively interacts with your invalid requests, eating up more CPU time on the device and therefore making the ammount of traffic needed to knock the victim off less.
A good (theoretical) example of required traffic to disable a network device can be seen in the following scenario:
Firewall Drop Rule: 3gbs
Syn Flood against any open TCP port: 0.5Gbs
Requests to an insecure, resource intensive, http search query: 10Mbs
As you can see, the more CPU cycles you force the target to use, the more effective the DDoS attack.