Developer testing is checking whether the code does what the developer themself thinks it should. QA testing is checking whether the code does what the customers / users / rest of the world thinks it should.

It’s a lot faster and easier than it used to be. Things like xUnit in the .net world make setting up tests friction free to the point where I question a codebase that doesn’t have some kind of basic unit tests. It doesn’t make mock testing or integration testing easier but I would argue if you know the base code and logic is sound those tests are less relevant.

One thing I found is that if testing is easy, your code structure does change a bit to aid with a “test first” approach and I don’t hate it. I thought it made me slower but it doesn’t, it ensures that when all the ground work is finished, the gnarly part of wiring everything up goes much faster.