getLatestBlockEntityId example

// Enable layout builder.     $this->drupalGet(static::FIELD_UI_PREFIX . '/display/default');
    $this->submitForm(['layout[enabled]' => TRUE], 'Save');
    // Add a block to default layout.     $this->drupalGet(static::FIELD_UI_PREFIX . '/display/default');
    $this->clickLink('Manage layout');
    $assert_session->addressEquals(static::FIELD_UI_PREFIX . '/display/default/layout');
    $this->addInlineBlockToLayout('Block title', 'The DEFAULT block body');
    $this->assertSaveLayout();

    $this->assertCount(1, $this->blockStorage->loadMultiple());
    $default_block_id = $this->getLatestBlockEntityId();

    // Ensure the block shows up on node pages.     $this->drupalGet('node/1');
    $assert_session->pageTextContains('The DEFAULT block body');
    $this->drupalGet('node/2');
    $assert_session->pageTextContains('The DEFAULT block body');

    // Enable overrides.     $this->drupalGet(static::FIELD_UI_PREFIX . '/display/default');
    $this->submitForm(['layout[allow_custom]' => TRUE], 'Save');