The filling order of atomic orbitals follows a specific pattern dictated by the Aufbau principle, which states that electrons occupy the lowest energy orbitals available before moving to higher energy levels. While it is true that the 5d block comes after the 4f block in the periodic table, the actual filling of orbitals depends on their energy levels and the order of increasing energy.
In the case of the transition metals, the 4f and 5d orbitals have similar energies, but the 4f orbitals are filled before the 5d orbitals. This is because the 4f orbitals have a slightly lower energy due to shielding effects from the filled 5s and 5p orbitals. The filling order of the orbitals is determined by the effective nuclear charge experienced by the electrons in each orbital, as well as the electron-electron repulsions.
Therefore, even though the 5d orbitals technically have a higher principal quantum number (n) than the 4f orbitals, the 4f orbitals are filled first due to their lower energy levels. Once the 4f orbitals are filled, the 5d orbitals are then filled in the subsequent elements. This is why you may observe a partially filled 5d orbital before the complete filling of the 4f block in certain transition metal elements.