In an atom, the order of filling up of the orbitals is governed by

The order in which atomic orbitals are filled is determined by the Aufbau (building-up) principle, which prescribes filling electrons into the lowest available energy sublevels first to obtain the ground-state electron configuration [1]. In practice the Madelung (n+l) rule gives a useful energy-based mnemonic for that ordering in many-electron atoms, explaining the relative energies of sublevels (e.g., 4s vs 3d). Hund’s rule and the Pauli exclusion principle are complementary: Hund’s rule governs how electrons distribute among degenerate orbitals (maximizing unpaired spins) and Pauli’s principle restricts identical quantum numbers, but neither alone sets the overall energy-based filling sequence [2].

Sources

1. [1] https://www.chem.uci.edu/~unicorn/old/H2A/handouts/PDFs/LectureA6.pdf
2. [2] https://egyankosh.ac.in/bitstream/123456789/53148/1/BCHCT%20131%20Block%201E.pdf