Surveyor-64 is licensed under Apple's standard Licensed Application End User License Agreement, the same agreement the app links to in Settings.
Read the License Agreement (EULA)
PRO is sold and billed by Apple through the App Store, as a monthly subscription or a one-time lifetime purchase. Manage or cancel a subscription in your Apple Account settings: iPhone Settings › your name › Subscriptions. Use Restore Purchases in the app to bring PRO back on a new device.