How to test an application? api code example

Example 1: what to test in api

Request Payload has
ENDPOINT (Specific URL where we send our requests to a certain web service. )
METHOD (Get•Post•Put•Patch•Delete)
HEADERS(Represents the metadata of request
or response. Like basically we can verify
the object type we are sending etc  )
PARAMETERS(Parameters are to specifiying 
even more about the request we are sending 
like what specific resources needs to be returned etc.)
BODY(Body is where we sent or received the request we made)
For the repsonse 
Basically we are checking response body
to verify if request matches with response.
In the response we are verifying
(body, status code, header, response time,
 test structure of json against the given jsonSchema)

Example 2: what to test in api

Basicall we are checking response body
to verify if request matches with response.
After that we are writing SQL query
to double check if database matches.
We can say happy path if all pass