Mac turns off randomly, fans keep spinning.

It sounds like there may be an issue with the sensors on the main logic board. Assuming there are no signs of liquid or other damage internally, you may need to run a tool that gives you a better sense of the logic board condition. Are you able to boot into the operating system at all, or does it always fail on startup? If you can boot into an OS, download a tool like TechTool Pro and run a full hardware test. If you can’t successfully boot into an OS, try running the built-in Apple diagnostic test (hold the ‘D’ key on startup).

You can also attempt to isolate the problem through the method of elimination - try disconnecting non-essential components from the logic board one at a time and testing for the symptoms after each disconnection. If the symptoms persist even with all non-essential components disconnected from the logic board, you most likely have a logic board issue that would require deeper knowledge to resolve.