He initially planned to approach her irl until he eavesdrops on one of her conversations by chance. The ML himself was not actually in her phone. He adapted an existing banned virus from his family into a virtual boyfriend app. He indeed invaded her phone with a virus, but we do not know how much of that data he was able to see past their conversations and her clothing choices for the chibi character. After all, he probably could have sent the money to her directly instead of stealing it from a stranger.