I want to know how cations become atoms by obtaining electrons.From where do they obtain those electrons?

In electrolysis, the electrodes are subjected to a potential difference. Therefore, there is an accumulation of charge, that is electrons on the negative electrode (cathode) and at the anode, there is a deficit of charge, that is electrons.

As cations come in contact with the cathode, being positively charged, attracts an electron from the latter and becomes an atom.

Similarly, when the anions in the solution come into contact with the positively charged anode, they give electron(s) to the anode and become atoms.

Does this answer your question? :)