Most US civil servants are hired through a competitive process. Given positions, these people all had security clearances.
There are exceptions that are hired as political appointees though. Normally these are the politically shrewd players, but regularly phd brainiacs, albiet politically inclined.
Most US civil servants are good people, and well credentialed and are hard working folks.
> How are they trained?
Federal Law enforcement are trained at the FLETC locations [1]. Federal law enforcement training is generally considered vastly superior to local law enforcement.
> How are they preventing abuse of power?
Pretty much every federal system has strong auditing and logging capabilities, and internal affairs/Inspector General. It doesn't always mean you can catch a perpetrator misusing a system, but you do have the data to get them after the fact.
> What is the culture there?
This is really an organization by organization thing.
US Secret Service historically had the best mission culture.
DHS, allegedly, is more of a dog-eat-dog organization. Many former DHS folks told me it was not a good place to work. Personally, all the DHS people I met were qualified. They have some immense challenges in DHS and that stress is probably part of the problem.
> Under the prior administration, DHS acted brutally, in many cases. What happened to correct that culture?
Civil servants, like the military, take their marching orders from civilian management. When an administration takes over the federal government in a transition, they assign political appointees to the highest positions that may receive senate confirmation, if they pass it. Given President Biden, the Biden administration installed the current leadership.
When you swap the leadership, you do significantly impact the culture. However, if civil servants are not following orders of the SES management, they can get in big trouble, to include, losing their positions.