They don't actually. A member of Congress can't just ask the head of the NSA to tell them what's going on and expect the complete details. Each house has a Select Intelligence Committee who do in fact receive briefs on these subjects from time to time. The Speaker of the House and Minority Leader in the House serve as ex officio members of the House committee and the Majority Leader and Minority Leader of the Senate serve as ex officio members of the Senate committee.
When it comes to these votes on these issues where intelligence is discussed, members look to follow the lead of members of this committee for guidance on how to vote, as these members are barred from divulging the exact contents of the meeting. (Though it may be argued they could easily enter it into the public record and have immunity via the speech and debate clause.) This committee is also supposed to exercise oversight but that's often problematic because...
Once the Congress has passed these laws, the Executive Branch will often interpet the laws as they see fit, and because of the clandestine nature of all of this, there is currently no mechanism by which they must or do inform the committee on how they've interpreted the law. There is very little Congressional oversight of clandestine activities, and this has been the case historically except for very rare occasions where the lid has been blown off so to speak and public hearings were held. The oversight here is the weakest link and in the absence of strong oversight, people in power will continue to expand and condense their power in Washington as history has shown.
When it comes to these votes on these issues where intelligence is discussed, members look to follow the lead of members of this committee for guidance on how to vote, as these members are barred from divulging the exact contents of the meeting. (Though it may be argued they could easily enter it into the public record and have immunity via the speech and debate clause.) This committee is also supposed to exercise oversight but that's often problematic because...
Once the Congress has passed these laws, the Executive Branch will often interpet the laws as they see fit, and because of the clandestine nature of all of this, there is currently no mechanism by which they must or do inform the committee on how they've interpreted the law. There is very little Congressional oversight of clandestine activities, and this has been the case historically except for very rare occasions where the lid has been blown off so to speak and public hearings were held. The oversight here is the weakest link and in the absence of strong oversight, people in power will continue to expand and condense their power in Washington as history has shown.