Declare a subresource

// src/App/Entity.php
namespace App\Entity;
use ApiPlatform\Metadata\ApiResource;
use ApiPlatform\Metadata\Get;
use ApiPlatform\Metadata\GetCollection;
use ApiPlatform\Metadata\Link;
use ApiPlatform\Metadata\Post;
use Doctrine\ORM\Mapping as ORM;

#[ApiResource(
    operations: [new Post()]
)]

#[ApiResource(
    uriTemplate: '/companies/{companyId}/employees/{id}',

    uriVariables: [
        'companyId' => new Link(fromClass: Company::class, toProperty: 'company'),
        'id' => new Link(fromClass: Employee::class),
    ],
    operations: [new Get()]
)]
#[ApiResource(
    uriTemplate: '/companies/{companyId}/employees',
    uriVariables: [
        'companyId' => new Link(fromClass: Company::class, toProperty: 'company'),
    ],
    operations: [new GetCollection()]
)]
#[ORM\Entity]
class Employee
{
    #[ORM\Id, ORM\Column, ORM\GeneratedValue]
    public ?int $id;
    #[ORM\Column]
    public string $name;
    #[ORM\ManyToOne(targetEntity: Company::class)]
    public ?Company $company;
    public function getId()
    {
        return $this->id;
    }
}
#[ORM\Entity]
#[ApiResource]
class Company
{
    #[ORM\Id, ORM\Column, ORM\GeneratedValue]
    public ?int $id;
    #[ORM\Column]
    public string $name;
}

// src/App/Playground.php
namespace App\Playground;
use Symfony\Component\HttpFoundation\Request;
function request(): Request
{

    return Request::create('/companies/1/employees', 'GET');
}

// src/DoctrineMigrations.php
namespace DoctrineMigrations;
use Doctrine\DBAL\Schema\Schema;
use Doctrine\Migrations\AbstractMigration;
final class Migration extends AbstractMigration
{
    public function up(Schema $schema): void
    {
        $this->addSql('CREATE TABLE company (id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, name VARCHAR(255) NOT NULL);');
        $this->addSql('CREATE TABLE employee (id INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL, company_id INTEGER DEFAULT NULL, name VARCHAR(255) NOT NULL, CONSTRAINT FK_COMPANY FOREIGN KEY (company_id) REFERENCES company (id) NOT DEFERRABLE INITIALLY IMMEDIATE);');
        $this->addSql('CREATE INDEX FK_COMPANY ON employee (company_id)');
    }
}

// src/App/Fixtures.php
namespace App\Fixtures;
use App\Entity\Company;
use App\Entity\Employee;
use Doctrine\Bundle\FixturesBundle\Fixture;
use Doctrine\Persistence\ObjectManager;
use function Zenstruck\Foundry\anonymous;
use function Zenstruck\Foundry\faker;
use function Zenstruck\Foundry\repository;
final class BookFixtures extends Fixture
{
    public function load(ObjectManager $manager): void
    {
        $companyFactory = anonymous(Company::class);
        $companyRepository = repository(Company::class);
        if ($companyRepository->count()) {
            return;
        }
        $companyFactory->many(1)->create(fn () => [
            'name' => faker()->company(),
        ]);
        $employeeFactory = anonymous(Employee::class);
        $employeeFactory->many(10)->create(fn () => [
            'name' => faker()->name(),
            'company' => $companyRepository->first(),
        ]
        );
    }
}