If you delete the app from your device, then this also deletes all the drawings stored under the "On This iPad" location. The drawings under "iCloud Drive" are not deleted and they remain accessible in the "Concepts" folder in your iCloud Drive.

If you have lost drawings stored only on the device, you can restore your device from iCloud Backup and that will restore your drawings.

If you do not have a backup of your device and your drawings were only saved locally on the device then unfortunately your drawings are lost.