Basement walls: Are voids in masonry normal?

Good evening.

Our basement is now built, but has not yet been horizontally mortared; otherwise, the bricks were just pushed together, although some stones have larger gaps between them (2cm?), is that normal?

I can't quite follow you right now. Do you mean the "gaps" between the stones, so to speak the butt joints? They are filled with mortar.

But how is the wall sealed further?

Double-layer bitumen coating with fabric insert should be able to seal it in the end. ;)