Drupal - 8 - How to programmatically add terms to an existing Node

This is the correct long form, as in the comment of mradcliffe:

$node->get('abc')->appendItem(['target_id' => 123]);

You don't need to load the term, use the term id.

You can use shorter versions:

$node->abc->appendItem(['target_id' => 123]);
$node->abc[] = ['target_id' => 123];
$node->abc[] = '123';

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