Does a container volume sync

Hi, i have a usb stick mounted to my rpi and want to use it for a docker
I have two options
a) mount it from the host enviroment or
b) mount it within a container

My question is for scenario a).
When attaching a volume outside the container to a folder inside the container,
Is content then first written inside the container and then synced to the outside (host) or directly written to the redirected volume on the host ?