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Create the test scripts in Assertion that will verify the fetched data from the application against an actual service call. The assertions are made with Citrus Framework (Detailed documentation: https://citrusframework.org/citrus/reference/2.8.0/html/index.html).
The framework allows the usage of JPath, which is helping to fetch the needed data from the REST service response.
SeleniumRequest / AppiumRequest
In that part of the project, it's the Software vendor that has to fill in the support code to fetch the data from their SUT.