Why does the 4s sublevel fill up before the 3d sublevel? What principle is that based on?

According to the Aufbau principle, the 4s orbital is filled first instead of the 3d orbital

Each electron shell and its orbitals have a specific energy level.

The farther the shell is from the atomic nucleus, the higher the energy level.

This energy level is expressed in the form of electron configurations.

Writing electron configurations starts from the lowest to the highest sub-shell energy level. There are 4 sub shell in the shell of an atom, namely s, p, d and f.

Charging electrons in the subshell uses the following sequence:

1s², 2s², 2p⁶, 3s², 3p⁶, 4s², 3d¹⁰, 4p⁶, 5s², 4d¹⁰, 5p⁶, 6s², etc.

According to Aufbau, electrons occupy orbitals of the lowest energy level  

So the 4s orbital has a lower energy level than the 3d orbital, so the electrons fill the 4s orbital first