Is an online relationship with a young man, which caused the emission of seminal fluid without the loss of the hymen, considered zina (fornication/adultery)? And does Allah’s saying, "The fornicator/adulterer shall not marry except a fornicator/adulterer or an idolater," apply to me? What should I do to repent to Allah? And would his marriage to me be forbidden?

Emotional relationships between a man and a woman who are not married to each other are forbidden outside the framework of a valid marriage. What occurred is considered metaphorical zina (fornication/adultery) because it is a means to it, but it does not entail the rulings of actual zina. You must repent, cut off the relationship with the young man, and block all pretexts that could lead to fitna (temptation/strife), while seeking help for steadfastness in repentance. Marriages based on online acquaintanceships generally end in failure. The girl should investigate anyone who proposes marriage and seek Allah's guidance (istikharah).