For the elements with large atomic numbers, we prefer to write the electronic configuration taking the help of inert elements near to that element. And for this, we may need knowledge about every element of the periodic table which is quite difficult.
So in this context, we take the help of half-filled and full filled orbitals. I mean the electronic configuration is done by fulfilling the electrons in the orbitals either fully or half to get a stable configuration. Since f orbital can hold 14 electrons but here we can't make it full so make it half i.e. 7 electrons. Now we have 3 electrons to arrange.
And here in d orbital, we can neither make it full nor half filled. So, we got to s and make it fulfilled and arrange the remaining electron in d orbital.
You will learn about this more in grade 11. Hope this may have helped you somehow.