What's the difference between a sandbox and a production project?
A sandbox project is a fully isolated test environment: payments run through Stripe's test mode so no real money moves, you can assign fixed one-time codes to test contacts so nothing is actually emailed or texted, users can be hard-deleted, contact methods can be marked verified directly, and request/response body logging is on by default. A live project flips each of those — real money, real codes, suspend-only, body logging off.
You choose sandbox at project creation and it can never be changed, so create separate projects per environment.