Local autostart file is missing, general autostart file throws error, direct execution of file goes well

Question

I try to make a script startup via an autostart file. But I run into two problems which I do not understand.

1. Why am I missing the ~/.config/lxsession directory. It is just a fresh raspbian installation (Raspbian Stretch Desktop + recommended software November 2018 version)
2. When I add the script to /etc/xdg/lxsession/LXDE-pi/autostart and run it manually with source. It says it can not find the command in my startup script. But when run directly (so not via the autostart file) it does not throw this error.

Are these problems related to my lack of knowledge upon the autostart file or is there a deeper source of knowledge required to understand these problems. Which resources should I dive into, in order to understand these problems. So I can resolve them.