What is the ruling on engaging in sexual relations through written chat conversations, sending inappropriate pictures, and completing the sexual conversation via chat until ejaculation? Is this act considered unlawful sexual intercourse (zina) or is it in the ruling of unlawful sexual intercourse, and how can one atone for it?

What you did, following the footsteps of Satan, is something Allah Almighty warned against in His saying: {And do not follow the footsteps of Satan. Indeed, he is to you a clear enemy} [Al-An'am: 142]. Such actions are reprehensible matters; they are not fornication that necessitates a prescribed punishment, but rather a means to it. Avoiding such means is a religious requirement, and repentance is their expiation. It is advised to stay away from Satan and his whispers, and to benefit from guidance towards righteousness. The internet and social media are a double-edged sword; if not used in a way that pleases Allah, it is better to avoid them.