If they don’t testify, they can put people in contempt of congress which can be enforced by the Sergeant at Arms.
The Sergeant at Arms as I wrote. They are part of congress. They are not part of the executive branch
Under this process, the procedure for holding a person in contempt involves only the chamber concerned. Following a contempt citation, the person cited is arrested by the Sergeant-at-Arms of the House or Senate, brought to the floor of the chamber, held to answer charges by the presiding officer, and then subjected to punishment as the chamber may dictate (usually imprisonment for punishment, imprisonment for coercion, or release from the contempt citation).[18]
The sergeant at arms of the United States House of Representatives is an officer of the House with law enforcement, protocol, and administrative responsibilities. The sergeant at arms is elected at the beginning of each Congress by the membership of the House.
Who enforces those subpoenas? Who enforces the contempt charges? Trump is in charge of all of the enforcement mechanisms.
The Sergeant at Arms as I wrote. They are part of congress. They are not part of the executive branch
https://en.wikipedia.org/wiki/Contempt_of_Congress
https://en.wikipedia.org/wiki/Sergeant_at_Arms_of_the_United_States_House_of_Representatives
So, one guy is going to go around arresting top officials of a dictatorship?