8 - How to programmatically add terms to an existing Node
This is the correct long form, as in the comment of mradcliffe:

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


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

You can use shorter versions:

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