So tester’ duty is to make sure the software application is bug free or defect free by implementing various scenarios with testing techniques, don’t u agree? pls. comment

@Hubot-0x58 right, but how would you define a testing role?

Testing engineer collaborates with programmers to make manual tests that test functionalities. It's an ongoing process. They also make automated tests, especially robustness tests, that repeat simple tasks many times. They also collect test data, organise it and present it.
Testers manually perform tests.

Purpose of these tests is to find unwanted behaviour, without looking at the code.

Unit tests and other integration tests are mainly created by developers, but there are organisations which have developers dedicated to creating these tests. Same goes for performance and resource usage tests.