The short answer is anywhere you want.
JavaScript has had a lot of different stages in its life and a lot of different types of people using it. Which is probably why most tools these days are highly configurable, to allow for personalization (customization.)
Even Jest itself shows these signs. For instance the test matcher will look for tests in either __tests__ folders or with files that are contain .spec or .test.
Or as per their docs in a visual manner:
├── __tests__
│ └── component.spec.js # test
│ └── anything # test
├── package.json # not test
├── foo.test.js # test
├── bar.spec.jsx # test
└── component.js # not test
With regards to fixtures and other test files the answer is the same, there is no one way to do it.
Pick what works for you.
I recommend for the __tests__ structure placing fixture data close to the test thats using it, and if multiple tests need access then move it further up the project until its in a common place.
My preference is a tests folder to keep the tests, fixtures, and testing separate to the src code.