I’m looking for a way to obtain an Observation from a Device that is associated with a patient.
For example, one patient has a thermometer associated with him and I want to obtain an Observation with its temperature.
Is it correct to create a ServiceRequest where the Subject is the Patient and the performer is the Device associated with the patient?
Yes, that would be appropriate
Which FHIR resources we have to use to get this information’s? and can we use the Patient Identifier as the key/unique attribute to collect the details from subsequent resources?
Observation would be the resource you’d use. As for how to search, that depends on what the search capabilities of the server you’re hitting are. If you’re creating your own server, yes you can retrieve an Observation by searching on Patient.identifier if you use chaining.