SMEs apply for different tasks such as Job Task Assessments (JTA), Objective Domain (OD), exam item writing, item selection, practice item writing, renewal assessment writing, exam item sustained engineering, and item scrubs. SMEs can be a mixture of internal and external people.
Typically you are asked for your experience in particular areas and a list of names is submitted and then from that list the exam owner makes a selection.
There used to be a SME database Help Us Create Exams and Learning Content! Join Our SME Database Today! | Microsoft Community Hub but that no longer exists, instead Join the LinkedIn group Microsoft Worldwide Learning SME Group for Credentials Microsoft Worldwide Learning SME Group for Credentials | Groups | LinkedIn
Microsoft use a selection of vendors to manage some of these tasks. Lookout for posts on LinkedIn from people who work for GrandMasters e.g., this one PL-900 OD