The NAS or home server operating system unRaid is installed on a USB stick and also booted from there. All other data such as Docker containers or virtual machines are not stored on the USB stick. This data is stored on the array and the cache.
The unRaid boot USB stick must therefore really only be sufficient for the unRaid installation. For this reason, the requirements are modest: the USB stick must be at least 4GB in size, and the maximum supported size is 32GB. A small, reliable stick from a well-known brand is all you need for an unRaid boot device.