How does the Serjeant-at-Arms get their job?
The Serjeant-at-Arms carrying the Mace.

DPS Auspic
Description
The Serjeant-at-Arms carrying the Mace in the House of Representatives. Behind her is the Speaker of the House of Representatives.
Permission should be sought from DPS AUSPIC for third-party or commercial uses of this image. To contact DPS AUSPIC email: auspic@aph.gov.au or phone: 02 6277 3342.
Thank you for your question, Michael.
As well as elected members of parliament, there are also non-elected employees who work for the Parliament.
One of the most well-known positions in the House of Representatives is the Serjeant-at-Arms. Like all parliamentary employees, this person had to apply for the position and be successful at a job interview.
The Serjeant-at-Arms will usually have worked in other positions at Parliament House and have a lot of experience with how the Parliament works.