No, that is only part of the way they go about it. They also survey businesses.
Obviously they can not survey every business, so it is just a statistical sample extrapolation.
Like I said, the only accurate count of jobs + self employed is how many people are paying social security tax. All earned income is subject to SS tax. Then divide that by adults of working age counted in the Census data and you will know the true picture.
Admittedly that number will show high unemployment because some people do not want to work at a job. But those people certainly should not be excluded from the ranks of unemployed.