Per request, the package can determine the "current" tenant. This is done by a TenantFinder. The package ships with a DomainTenantFinder that will make the tenant active whose domain attribute value matches the host of the current request.
To use that tenant finder, specify its class name in the tenant_finder key of the multitenancy config file.
// in multitenancy.php/*
* This class is responsible for determining which tenant should be current
* for the given request.
* This class should extend `Spatie\Multitenancy\TenantFinder\TenantFinder`
*/'tenant_finder' => Spatie\Multitenancy\TenantFinder\DomainTenantFinder::class,