Session package implements a session service, PSR-15 session middleware, and a flash message service which helps use one-time messages.

You can access session data through SessionInterface.

/** @var \Yiisoft\Session\SessionInterface $session */
$myId = $session->get('my_id');
if ($myId === null) {
    $session->set('my_id', 42);
}

In case you need some data to remain in session until read, such as in case with displaying a message on the next page, FlashInteface is your friend:

/** @var Yiisoft\Session\Flash\FlashInterface $flash */

// request 1
$flash->set('warning', 'Oh no, not again.');

// request 2
$warning = $flash->get('warning');
if ($warning !== null) {
    // do something with it
}