In Drupal 9 poll and poll choice both are separate entity.

Create a Poll Entity :

/* Poll will be created here. */
$poll = Poll::create(array(
'id' => 'poll',
'uid' => '50',
'question' => $title,
'anonymous_vote_allow' => '1',
'cancel_vote_allow' => '0',
'result_vote_allow' => '1',
'status' => '0',
));

Create a Poll Choice Entity :

/* Choices will be added here. */
$choices = PollChoice::create(array(
'id' => 'poll_choice',
'uid' => '50',
'choice' => $data
));
$choices->save();
$choice_id = $choices->id();

Adding created choice to the poll :

$poll->choice->appendItem($choice_id);
$poll->save();