Yhyjyk

We know that the whole development team is responsible for the quality of the product. If only the testers test, there can be no question of shared responsibility. Of course, the testers in the team still need the ability to add value in their specialty. They bring with them valuable knowledge and skills. Nevertheless, all team members can and should contribute to the quality of the product, for example:

• Developers can decompose the software into small, independently
  testable components.


• Developers can help automate test cases.


• With their experience and know-how, testers can alert developers to
  potential program vulnerabilities or perform risk analysis. So
  
  that it is ensured at an early stage that some errors do not even
  
  occur.


• All team members can and should work with the user to ensure early on
  that the right thing is being developed.

Classic testing is Quality Assurance

Agile Testing is Quality Engineering

Quality Assurance

Traditionally most testing as done at the end of the development. It asks if the product that has been developed meets the requirements, both explicit and implicit.

Quality Engineering

This field is primarily concerned about automation and automated tests.
It breaks out testing into:
Unit testing
Integrated Testing
Automated UI
Exploratory, Performance & Security

Over time this leads to the following situations:

Classic Testing - the product is built, the developer considers it is working but Quality Assurance has to convince them and others when there are issues. Issues are frequently found late in the process with little time for quality fixes. Fixes are expensive and often affect production. This is common in Command and Control and Waterfall environments.

Agile Testing - the goal is to shift testing left and test as early as possible. Ideally many tests are written first and fail without the application code to support them. Most tests are unit tests written by developers to ensure components work. This makes them executable specifications and when the application code is written that makes them pass, then you are good to go.