Nero Video Manual could be found in Nero Video application. 

Open Nero Video, click KnowHow in the top right corner. 

In the drop down menu, click Download Manual.

Start from Nero 2019, 'Live Guide' is available via 'Nero KnowHow PLUS'.

Switch on the 'Live Guide' toggle on the header bar of Nero Video, Live Guide will give you context sensitive Manual, FAQs and Tutorials of the current screen of Nero Video. 

When you navigate to other screens, the content in Live Guide will automatically update to show the information of the current screen.