The iPhone XS series has some of the same problems as the iPhone X because of the similar stacked motherboards. However, The XS series has a better design in terms of how the pads and vias attach to the lines. Instead of small vein like traces, there are thick traces. These allow a much more secure via repair because a lot more solder can be applied. Because of this, motherboard repair for the XS series seems a much more reliable option then motherboard repair for the X series.
In this phones case, when you try to make a phone call sound does not work with either ear speaker or loud speaker or any mics. Sound does work for other functions such as voice memo and audio during video record however.
Here you can see that the two stacked motherboards have already been split. These are the missing pads. Some of them are ground are are not needed for the circuit to be complete:
Here are the voltage lines that are missing. They must be repaired for full functionality of the phone:
Here you can see some of the missing pads that are required. They have already been scratched to reveal the traces underneath:
Here you can see the solder added to the vias. A nice thick blob that goes at least as high as the neighboring pads is best. Unlike the iPhone X, a jumper wire is not needed because so much solder is able to be applied. After this step, reball the board and put back together:
Here the boards are being put back together. Coins are applied to give a small amount of pressure and ensure the pads come together:
When finished, sound is working properly and you have a usable phone!