Software-Defined Wide Area Network (SD-WAN) is a more flexible WAN architecture that can take advantage of multiple hardware platforms and connectivity options. The controlling software works with any networking hardware. An organization can set up an SD-WAN using off-the-shelf hardware rather than specialized hardware. This makes SD-WANs cheaper, more flexible, and more scalable than traditional WANs.
Thinking of a desktop computer that run a proprietary operating system and a computer that works with varied options of operating systems. For more specific, it is Linux, how compatible it is to a various choice of operating system.
While this does not exactly represent the usage of SD-WAN, a similar principle applies: SD-WAN is decoupled from the hardware components, giving organizations more choices of hardware.