How does NextDNS work behind the scene?

I'm not their employee or anything. but while each user can choose which lists to enable, those are from preselected collections. It's possible to compile a combined list that contains deduplicated domain as key with the value being combined flags of whether that domain exists in a collection member. So let's say you have 

List A : example.com doubleclick.net

List B : doubleclick.net google.com

The combined blocklist will be :

example.com : 10

doubleclick.net : 11

google.com : 01

Then if users have config C that enables both list, config D that only enable list A and config E that only enable list B, then the config list will be :

config C : 11

config D : 10

config E : 01

It's trivial to then use binary AND between the combined list and each config entry, any non-zero value means it's blocked, ie, if we want to check if config C blocks example.com, then it's 11 AND 10 which return 10, indicating the domain is blocked, while against config E, 10 AND 01 is 00 which means example.com isn't blocked.

Hash lookup takes constant time, so does the comparison operator, so CPU-wise, this solution is scalable, and even storage-wise since most lists would have significant overlap anyway (in the example, the combined blocklist only has three entries instead of 2 + 2).

I'm sure NextDNS implement an even more sophisticated algorithm than this, but as you can see even with basic DB you can get O(1) complexity.