Content from two sections but one of which is related to a specific category
You would do such things by first getting the entries IDs and then do a query to get all entries with these IDs:

{% set entryIdsA = craft.entries.section('SectionA').ids() %}
{% set entryIdsB = craft.entries.section('SectionB').relatedTo('CategoryC').ids() %}

{% set allEntryIds = entryIdsA|merge(entryIdsB) %}

{% set allEntries = craft.entries.id(allEntryIds).order('mySortByField desc') %}