SSD upgrade MacBook Pro (15-inch, 2019)

Apple SSDs have been proprietary with the exception of certain models. To find which ones visit OWC. Otherwise, they are soldered flash memory on the motherboard. Furthermore, even were the SSD replaceable, if you replaced it, then you would void all service on the computer by Apple, including warranty service. It is advisable that you not open the computer for any reason other than to replace user-upgradeable components. Typically, that means replacing memory modules in models that use memory slots. Current models like yours do not have slots. Apple does not want the user tinkering with the computer's innards. If you like tinkering with the innards, then you bought the wrong computer.