Hi Aziz. That is a bit broad question. A test engineer develops a suite of tests by writing them in a testing framework of their choice. I use pytest most of the time. Basically I write the tests as functions in which I assert that a test result is in accordance with my expected result. For example:
assert request.status_code == 200

If the status code is not 200, this means the test will fail because the assertion is wrong. A group of tests is then executed in an automated fashion by a CICD server like Jenkins or Github Actions, whenever anything is committed to the main branch, or whichever trigger you configured to activate the CICD pipeline.