Why do electrons enter the $4s$ orbital before entering the $3d$ orbital?

According to the Aufbau principle, the lower energy orbitals are filled first after higher energy orbitals are filled.

The energy of the orbital can be determined by the (n+l) rule:

For $3d$ orbital:

,$\mathrm{n}=3\mathrm{and}\mathrm{l}=2$

$\mathrm{n}+\mathrm{l}=3+2=5$

For $4s$ orbital

$\mathrm{n}=4\mathrm{and}\mathrm{l}=0$

$\mathrm{n}+\mathrm{l}=4+0=4$

Therefore, it is clear that the $4s$ orbital has lesser energy as compared to $3d$ orbital.

So, the electrons enter the $4s$ orbital before entering the $3d$orbital.