1. when srs come from client , on the basis of tht document
use cases are generated mainly by developer,then on basis of
this scenarios are generated , then test cases are formed
.it is not necessary tht scenarios are made , many project
requires only test cases.

Test Scenario: Identifying the all possible areas to be 
tested (What)

Test Case: describes an input action and expected result to 
ensure that a certain feature works correctly as per the 
specifications.

Test Scenarios: End to End flows mainly derived from the Use cases developed by the Developer and it also includes any possible missing paths the system can take from start to finish.

Test cases: These are much simpler checks (usually single checks) e.g. 
click on the log-off link --> user should be logged out.