Laravel 8 from scratch - How to make a variable to use in routes?

Sorry if this question is a bit basic. Been out of programming for a couple years and watching "Laravel 8 from scratch" to get back into it. In episode 8 at 3:10 he is editing routes in web.php and he says "lets move this into a variable", then he proceeds to use a refactoring in phpstorm to create the variable, and says if you don't have phpstorm then do it manually. I am using VS Code and am unsure the appropriate way to do this manually.

I know that I could make a .php file and stick it in the app folder or in a controller, but not sure if this would be breaking convention, also not sure if it's acceptable to do this inside the web.php file. So my question is, what would be the conventional way to create a variable like he did for use in the web.php routes file manually?