How to set a 'scratch file' destination in FCPX 10.6.5?

The simplest way is to create a library on the T7. Open the library and import your media, either into the library itself or to an external location. If you need to collaborate or use the media with other applications it’s better to work with external media.

For an excellent introduction to FCP start with Steve Martin’s 60 minute intro https://www.youtube.com/watch?v=HqUP7Zgeuck. For a more in depth introduction use the IzzyVideo tutorials. The version is a bit dated but the principles are the same. 

Scratch disk is not used in FCP. The project is the edited sequence. The library is the data container, which can also include the media, though it doesn’t have to. Could you explain what you’re trying to do?