I suspect that anyone who has knowledge of how this dragnet has been implemented is under NDA, but reading articles such as [0] make me curious as to how Internet Connection Records (ICRs) are being collected and stored. Is it as simple as DNS requests to the ISPs DNS servers are logged into a database?
"It is the most intrusive and least accountable surveillance regime in the West" - Edward Snowden (@Snowden) November 4, 2015
[0] https://www.wired.co.uk/article/internet-connection-records-ip-act