If you are a student or a researcher on a budget, you don't need Cobalt Strike to learn command-and-control (C2) concepts. There are several that are respected in the industry:

Searching for "free" or "cracked" versions of Cobalt Strike is one of the highest-risk activities in cybersecurity. Because the tool is so powerful, cybercriminals often use the promise of a free version to infect the downloader's own system. Cobalt Strike 4.4 Cracked: Is It Worth The Risk? - Ftp

When searching for a "Cobalt Strike download file free," you will likely encounter numerous websites offering cracked or "leaked" versions.