How do I set the unit in the url?

You are in the right track of using middleware similar to localization. Here is the general gist:

• Create a SetUnit Middleware:

class SetUnit
{
    public function handle(Request $request, Closure $next): Response
    {
        // if user has no unit, abort
        if (!$request->user()->unit) {
            abort(403, 'You are not allowed to access this resource.');
        }

        $requested_unit = $request->route('unit');
        $users_unit = $request->user()->unit->slug; // name

        URL::defaults(['unit' => $users_unit]); // auto-fills unit when using route()

        // if current unit is not equal to user's unit, redirect to the user's unit dashboard
        if ($requested_unit !== $users_unit) {
            return redirect()
                ->route('unit.dashboard', ['unit' => $users_unit])
                ->with('error', 'Redirected to your unit.');
        }

        return $next($request);
    }
}

• Register your middleware in $middlewareAliases:

'set.unit' => \App\Http\Middleware\SetUnit::class,

• And the route something like this:

Route::prefix('{unit}')
    ->name('unit.')
    ->middleware(['auth', 'set.unit'])
    ->group(function () {
        // All routes inside...
        Route::get('/dashboard', function () {
            dd(route('unit.dashboard'));
            // you can see that {unit} fragment is automatically added.
        })->name('dashboard');
    });

Hope it helps.

you could put the unit on the end of the URL and make it optional.

If the controller that receives the request does not actually need the unit name then it makes no odds whether it is there or not.

So then the only bit you need to take care of is adding the unit to the end of each link.

You could have a 'current unit' for the user in session and then add it into each route when you create the href or form action.

To further illustrate, see my site. here is an example url

https://speakernet.co.uk/talk/4583/a-garden-within-doores

You will note the resource id (4583) is in the URL followed by a slug that exists in my case purely for SEO reasons. The controller never looks at that part of the URL.