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. The USB stick must be at least 4GB in size, the maximum size is 32GB. Here are some recommendations for unRaid boot USB sticks: