Underscores is primarily built so that you put your styles in style.css. This stylesheet is compiled from the main SASS file at sass/style.scss. Additionally, woocommerce.css is compiled from sass/woocommerce.scss.
If you want to take advantage of SASS yourself, you can either add your styles to sass/style.scss, or import a file into it that has your styles. You can see that it already imports some other files from the sass/ directory, as an example.
However, you should not enqueue the .scss file. Browsers cannot read SASS, so the .scss file needs to be compiled into the style.css file. Underscores does not provide any tools for doing this though, so you would need to add your own build tools for compiling SASS so that sass/style.scss can be compiled to style.css. If you’re previous HTML site was built with SASS, then whatever process you used to compile the stylesheet would also work here.