Software needs to work when the expected path changes. We test the important journeys, permissions, validation, calculations and integrations rather than only checking whether each screen loads.
The exact testing approach depends on the project, but it can include automated tests, browser testing, API checks and manual review with realistic data. Your own acceptance testing is also important because the people who understand the real process will notice details that a generic test cannot.
Going live may involve more than deploying code. Existing users may need accounts, old records may need to be moved, integrations may need live credentials and staff may need a clear plan for changing from the old process to the new one.
We prepare that transition as part of the project. After launch, monitoring and error reporting help show whether the application is behaving as expected and where something needs attention.