relatedTo() only on one field?
Haha. I figured it out. Leaving the answer here in case anyone else runs into the same problem:

$product = Entry::find()-&gt;section('products')-&gt;relatedTo([
    'element' =&gt; $component,
    'field' =&gt; 'productComponents'
])-&gt;one();


You can pass an object/array to the relatedTo() method as described here:

Relations | Craft 3 Documentation