SSD upgrade MacBook Pro (15-inch, 2019)

Upgrading an SSD in a Mac with a replaceable SSD would not give you issues or not work provided the SSD is not proprietary to Apple. OWC sells such SSD upgrades for almost all Mac models with socketed SSDs.

Apple techs do not remove soldered SSDs and replace them. You buy a new motherboard. The old board is returned to Apple. Special soldering machines are used that can heat and suck solder off the board in order to remove the chip. That same device is used to flow solder onto the board without damaging the chip. It's unlikely Apple does anything like that given that the cost for the time and labor far exceeds the repair price.

And there is NO second empty "slot"? What do the people if they buy a second (2019) MBP an need more ssd place? External ....?????

This makes no sense. Please read, carefully, what you write before clicking on the Post button.

There is no empty slot. If you buy a second 2019 MBP but need more SSD storage, then buy it with a larger SSD or tack on an external drive.