Combine AND and OR in a ElementQuery
If I understand properly, you should be able to do this:
$entries = \craft\elements\Entry::find()
    -&gt;section('yourSection')
    -&gt;category(['and', 695])
    -&gt;relatedTo([698,698])-&gt;all();

Assuming category is the name of your relational field, you can use that to pass the and condition (though with one element only it wouldn't be needed) then relatedTo is an or by default (docs)
And as per your edit, pass and condition to your custom fied and or conditions to your relatedTo.
It would be pretty much the same in Twig.