A chain must consist of an equal number if inner links and outer links. You cannot connect an outer link to an outer link or an inner link to an inner link.
A normal master link or quicklink replaces one of the outer links and two pins. But you leave the corresponding inner link in place. Then you connect the two inner links on both ends of the chain by the quicklink. Therefore, no change in the chain length happens.
This is one of the many videos that show the procedure. You really just replace the outer link with the quicklink, but the chain remains the same:
The video thumbnail should illustrate the quicklink location on the finalized chain quite nicely (pointed to by the arrow).