Where did we get the evidence about the earths structure?

We can only directly observe the earth's surface and (through mines and wells) get a peek at structures less than half a percent of the way to the center of the planet.

We can observe natural phenomena that bring material up from farther down, such as volcanoes.

We can also use models to predict what earth must be like, knowing the gravity and mass and size of the planet.

We can also use information gained by the reverberations of large surface events to map out the interior of the planet by sonar. In the days of underground nuclear tests, and from modern-day earthquakes, we can see patterns of how compression waves travel and reflect from the interior.

The science involved in this methodology is incomplete and still evolving.