The vet will look at your horses eyes, check the gums/tongue for color, take a temp, check the heart, make sure it has current shots and a coggins. Fairly easy thing. He may take blood IF there has been any disease outbreaks near you. Generally if there isn’t, and vitals are normal, it will pass.

A health Certificate is easy! The vet is just looking for your horse to be healthy for shipping. He will take its temp and listen to his heart beat. He will note on the paper owners name, where the horse is being transferred too, color, sex, age, name, temp. When I have shipped TB’s in the past too and from tracks or breeding farms he just takes there temp and thats it just knowing I have a healthy herd.

Simple things.
Be in proper weight. No ring worm/other worms, hooves trimmed properly, Nice coat. And no illness’ like collic or anything like that.

If your horse would be able to sell fairly for over 1,500. It will probably pass.
hope I helped.