When you say someone is "heavily into" something, it means they are deeply involved and passionate about that activity or interest. There are several synonyms you can use to describe this level of engagement. For instance, you can say someone is "obsessed with," "enthusiastic about," "devoted to," "addicted to," "immersed in," or "passionate about" something. These words express a high level of commitment and focus that someone has towards a particular interest. Relating someone's level of involvement in this way can be useful in both personal conversations and professional communication.