Where is "ildasm" in Visual Studio 2019?

Try to use it directly from Developer Command Prompt 2019/2017

ildasm.exe is installed with the Windows SDK tools, located in C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6.1 Tools\ildasm.exe or somewhere similar depending on the version.

You can install the SDK from this website or by selecting it in the optional components during the installation of VS2019.