I think it uses claude code by default, it is literally the same thing, with different (better) interface.
> Which LLM are you?
> I am Claude, an AI assistant created by Anthropic. In this interface, I'm operating as "Junie," a helpful assistant designed to explore codebases and answer questions about projects. I'm built on Anthropic's large language model technology, specifically the Claude model family.
Jetbrains wider AI tools let you choose the model that gets used but as far as I can tell Junie doesn't. That said, it works great.