You 'burned' the Zorin OS .ISO file to the USB stick. So what you're seeing is normal.
The 'Zorin OS 16.2 Core 64bit' partition is what you boot from when you boot into the Live (ie: not installed) version of Zorin OS.
The 'writable' partition is the left-over space on the USB stick. The 'Zorin OS 16.2 Core 64bit' will save the logs (from each time you boot the USB stick) to this partition, you can delete those whenever you like.
If you want to get rid of all that, you'd delete those partitions, or zero the drive. Let's say the drive is /dev/sdc:
sudo dd if=/dev/zero of=/dev/sdc bs=512 status=progress
Then you can re-partition the USB stick to whatever you want, and put whatever you want on it.
Now, obviously, you cannot delete the partitions on the USB while booted from the USB, and if you don't have an OS installed on your computer already, you'll have to use another computer to delete the USB stick's partitions (or zero the USB stick), and 'burn' another .ISO file to the USB stick.