How do you email a boss about work done?

Any initial discussions with your boss should be done verbally. There's a lot to be said for a face to face or in these pandemic times a Zoom call.

It's a great way to put a framework to a task or pull together brainstorming ideas and address quick queries.

Updating your boss about the outcome or conclusion of a work assignment set by them is reasonable and a useful way to ensure you have a 'paper trail' of how things went, should you need it later.