Warning: The following steps are outlined to explain the process used by pirates. We do not endorse this. Attempting this may expose your Mac to security risks.
If a user ignores the warnings and attempts to download a cracked version of Photoshop CC 2015 for Mac, they would typically do the following:
They open the "Crack" folder, run a patcher application, and drag the "Photoshop.app" into the patcher window. The patcher modifies the executable code to bypass the license check.
This is the only "safe" way to run old, cracked software. You can install a virtual copy of macOS Sierra or Windows 10 using UTM or VMware Fusion Player (free for personal use). Inside that virtual machine, you can install Photoshop CC 2015. Because the software is sandboxed in the VM, your actual Mac remains safe. However, this requires 16GB+ of RAM and significant hard drive space.
If you need Photoshop for a project right now, the safest and only legal way to get it for "free" is the official trial.