The President can nominate anyone he wants to any cabinet position.
The Senate Majority Leader is the one who decides who will pass confirmation and be hired.
"On Thursday CNN released never-before-seen footage of Nancy Pelosi and Mitch McConnell inside the US Capitol on January 6, 2021 — the two people who refused to call in the National Guard prior to that day and during the rioting."
Trump didn’t clean out the DOJ.