Xubuntu is a Linux distribution and the foundational part of the operating system is the Linux kernel. The Linux kernel contains thousands of device drivers and the majority of them are open source, which allows kernel developers to modify them. To update these open source drivers, you only need to update the Linux kernel, which is one of the components updated through the Software Updater.
When hardware manufacturers do not release sufficient technical details for their hardware, it is not possible to create a complete open source driver, so the hardware may not have an open source driver or will have an open source driver with limited functionality. Some open source drivers are reverse-engineered to provide an alternative to the hardware manufacturer’s proprietary driver, but these will not be as feature complete.