yolo.scss

// In a file main.scss
@import 'plop';
@import 'hey';

// module _plop.scss
@mixin plop {
  background: red;
}

// module _hey.scss
.hey {
  @include plop;
}

// Conclusion
// -> the module _hey.scss can use everything defined in the module _plop.scss without importing it...
// -> looking at _hey.scss, the relation between the mixin plop and _plop.scss is far from obvious...