SDN and Conventional Switches – Compatibility Guide

OpenFlow specifically - but also generally applicable to all of SDN - doesn't change anything about how your physical device forwards packets. The hardware is the same, and the pipeline that already existed on that device is unchanged.

What OpenFlow does is move the logic for programming the packet matches and actions from on the device itself (the local firmware), to a process that is running on a different device, which may have a view of more than one network element (and thus may be able to make better decisions with a better view of the world - or not). So, while you may configure network routing at a single controller, the controller is then distributing that information to a lot of devices to do what they have always done - match, modify, and forward packets.

First note that all of these technical terms are subject to distortion by Marketing, so sometimes you have to read between the lines.

The term switch refers to a layer-2 device, so it is forwarding on layer-2 info. On an Ethernet this would be the 48 bit MAC. With MPLS it would be the Label. The term router officially refers to a layer-3 device, so it is forwarding on layer-3 info (the 32/128 bit IPv[46] address, unless you have a very strange network). That's the primary difference between the terms.

How each device actually implements that forwarding is partly controlled by the protocols defined to exchange the relevant data, and the engineering choices made by the device designer, and is mostly opaque to the outside. So, to get more detail on how a particular product does it's thing, you need to get more detail from the manufacturer than the legal department wants them to divulge, so under NDA (Non-Disclosure Agreement). Unless the device you care about is just a general purpose computer running Free or Open Source Software, in which case, you look at the source.