Who decides who holds that job?
There are a limited number of Supreme Court justices.
The President puts forward nominations, a Senate hearing takes place and the Senate then votes. A simple majority is all that is required.
Trump fell lucky in that three posts became vacant during his Presidency and a majority of like minded Senators pushed his nominations through. This allowed Trump to nominate not just a right wing justice on each occasion, but originalists such as Amy Coney Barret and Neil Gorsuch, who believe the constitution should be interpreted as it was understood to be at the time of its adoption.
There were already right wing justices on the SC such as the corrupt originalist Clarence Thomas. Trump's nominations ensured an overwhelming majority on the Supreme Court and sealed the fate of, for example, women's rights over their own bodies.
Trump has bragged numerous times of how he, through loading the SC with right wing justices, was responsible for the Dobbs decision which overturned Roe v Wade.