Both forms are grammatically correct (contrary to the insistence of some British grammar purists). The first one (“What happened?”) is the one most of us would likely ever to need in normal life. Use “did” when we knew something had happened but wanted more details.

When to Use happen or happens?

Similarly, 'even events...' is an additional information and hence verb will be used according to 'everything' therefore, 'happens' will be the correct option. Everything in life, even events that seem insignificant when they occur, happens for a reason.

How did it happen or how does it happen?

Feel free to just provide example sentences. how did it happen is used when the subject is already done... while how does it happen is used either it has happened or not. But it most likely refers to tbe future as if your wondering how does it feel...