This is a description about how do make very flat vias for double sided PCBs.
With these vias it is possible to even put parts above the via since they are almost as flat as the PCB copper layer.
The vias are produced by a wire of the same diameter as the hole. And pressed into the hole. In principle no soldering is required for the vias, they should make a stable connection just by the pressing.
A description can be found here: