After creating the tables in your DB. The next step is defining the relationships in each model. Details here: https://laravel.com/docs/5.5/eloquent-relationships#defining-relationships.
If you're just learning Eloquent you should start with simpler relations, but here we go.
Looking at the tables it seems these are the relationships:
User: hasMany routes() and hasMany locations(). I don't know why the locations table has an user_id though.
Route: belongsTo user(), belongsTo locationA() and belongsTo locationB().
For locationA and locationB you'll have to define a custom foreign key in the relation method. It's all in the documentation.
Location: belongsTo user(), hasMany routeA() and hasMany routeB().
Again, routeA and routeB are relations to the same model, but with different foreign keys. You can name the methods however you want to make them make sense in your business logic.
After all the relations are defined, or at least the ones you need, you can get the routes of the authenticated user like this:
// The locations are eager loaded for performance
$userRoutes = Auth::user()->routes()->with('locationA', 'locationB')->get();
foreach ($userRoutes as $route) {
$route; // the Route model
$route->locationA // The Location model with id matching location_id_a
$route->locationB // The Location model with id matching location_id_b
}