The disk will be queued for creation which occurs almost instantaneously, but it may not be mounted to the machine straight away depending on the machine's power state.
If you created the new disk when the virtual machine was shutdown then it will auto-mount on next boot. However, if the machine is already running you will need to mount the disk manually before it can be used within your virtual machine. Refer to the Mount a disk to a virtual machine how-to guide for the steps on how to achieve this.
You are given the option to have the disk formatted in a selection of file systems at the point of disk creation. However, you can also opt for a blank block device to which you can interact with using tools such as GParted from our ISO library, or in any other way you see fit from within the guest operating system; the choice is yours. If you wish to have Katapult format the disk for you post-creation then you still do that from the separate disk formatting wizard. For more information on that, refer to the format disk how-to guide.