Functional testing should only be used when directly navigating the page virtually through the testing environment. This is done in Drupal by a collection of Drupal methods and the Behat Mink web driver. Here are the resources I use when figuring out which of the multitude of methods available to me fit my needs:

Note: these links are for a development version of the Drupal api docs since the official site doesn't show all pages well.

• BrowserTestBase: This is the class all our Chado functional tests are based off of. As such all of these functions can be used via $this->METHOD().
• UiHelperTrait: This class helps with navigating pages and specifically forms. It is included by default in the browser base so all of these functions can be used via $this->METHOD().
• WebAssert: This class provides assertions specific to the page. These methods are accessed through the UiHelperTrait::assertSession() as follows $this->assertSession()->METHOD().
• FieldUiTestTrait: This class is specifically for testing fields through the UI. I have not tested this approach but want to save it here so I don't lose it. This trait would need to be added to your test class via a use statement.
• Behat Mink: Traversing Pages: This documentation shows you how to use the mink browser controller directly to find specific fields. These methods are accessed by using $this->getSession() to get a MinkSession or $this->getSession()->getPage() to get a TraversableElement.

Additionally, here are some examples of functional tests in Drupal that I'm using to piece all this together.

• EntityReferenceAdminTest: This is a functional test that walks through a variety of forms while testing the Drupal Entity Reference field.
• MediaThumbnailFormatterTest: This is a functional test that creates a Media Reference field programatically and then tests a content edit page, I think? But is from DRUPAL 9