In times of public emergency, the governor of a state, or the mayor of DC, can call up their National Guard units. The Defense Department then runs those NG units during the emergency.
Read the links below. What part of them don't you understand?
On 4 Jan 2021 the AP said:
WASHINGTON (AP) — Bracing for possible violence, the nation's capital has mobilized the National Guard ahead of planned protests by President Donald Trump's supporters in connection with the congressional vote expected Wednesday to affirm Joe Biden's election victory.
apnews.com
Also confirmed by CNN, on 4 Jan:
Acting Defense Secretary Christopher Miller on Monday approved a request from the mayor of Washington to deploy DC National Guard forces to the city to support local authorities during pro-Trump demonstrations scheduled in the city this week, a defense official told CNN.
www.cnn.com
Politico said today, 7 Jan:
"After the Pentagon on Wednesday approved an initial request from D.C. Mayor Muriel Bowser to send in more members of the D.C. National Guard — opting to mobilize all 1,100 of them — top Pentagon leaders huddled to decide whether to take additional action on Wednesday night, the official said. Miller and Army Secretary Ryan McCarthy ultimately decided to mobilize additional personnel due to the severity of the situation."
The deployment caps an intense few days in which D.C. and federal officials argued over whether and how to deploy U.S. troops in the capital.
www.politico.com