Definitely short and informal. You should have a legitimate reason to email (like, tell the person genuinely why you respect them or what about them you like), and a clear ask if there is one (asking for help, looking for a job, etc).

Don't talk about yourself, except if it's directly relevant to them.

I can easily tell when someone wrote 100 of them generically, vs when it's genuine, and I'll always respond to the genuine ones.

(And to be clear, I'm referring to the type of cold emails Zach is... people wanting a genuine connection. This doesn't apply to trying to sell something.)