How can I know if someone is accessing my Mac?

If you think your Apple ID has been compromised - Apple Support (CA)

If I understand what is meant by the term " shaker " you mean a Hacker ?

To gain access to this computer would required the individual to have direct physical access to this computer.

Then to have been give the Admin Account Password or access to the password you have stored elsewhere and Not On the Computer

Only then could they possible installed their Software on this computer.

Judging from your by-line " MacBook Pro 14″, macOS 12.6 " this is fairly new Apple Silicon Computer.

The added Security in Apple Silicon Computers Disables Booting from an External Device as the default setting

So this would guard against this possible way of the Hacker " shaker " for gaining access to this computer

Change security settings on the startup disk of a Mac with Apple silicon - Apple Support (CA)

Are things going on that you have not initiated? If so what exactly?

Keep in mind, as long as your Mac is powered up, things happen in background ... that's by design. But you have to understand the difference between a user-caused event and a system self maintenance event.

What is a shaker? Please be more specific as to precisely what is happening.