Code which anybody can view is called "source available". You aren't necessarily allowed to use the code, but some companies will let their customers see what is going on so they can better integrate the code, understand performance implications, debug and fix unexpected issues, etc. The customers would probably face significant legal risks if they took that code and started to sell it.

"Open source" code implies permission to re-use the code, but there is still some nuance. Some open-source licenses come with almost no restrictions, but others include limiting clauses. The GPL, for example, is "viral": anybody who uses GPL code in a project must also provide that project's source code on request.

What do you think the chances are that Microsoft would surrender the Copilot codebase upon receipt of a GPL request?