Jobs at small Sacramento companies nobody posts about - 26 weeks in

A friend who works at a local manufacturer says there are openings not advertised online. I do not usually post, so bear with me. They like a brief email with a résumé. What do you write in a cold email?

A short email with two sentences about what you can do and your résumé attached worked for me. I followed up after a week.

Visiting in person worked for a trades job. For office jobs, email was better.