Junie is an agent. The AI Assistant is the UI for agents (typically in a sidebar). You can switch to other agents if you want (such as Codex or Claude Code), and there are actually a ton that you can add with just a few mouse clicks.

Example: https://gist.github.com/rbehrends/4d9febe5c5c008961b309dd4a6...

The "Install From ACP Registry ..." allow you to use pi, OpenCode, Kilo Code, Cline, Cursor, Copilot (and a ton of other agents that I've never heard of) if you want.