• Create a ContentVersion record. This creates a ContentDocument record automatically if you leave the ContentVersion.ContentDocumentId field blank. (https://developer.salesforce.com/docs/atlas.en-us.api.meta/api/sforce_api_objects_contentdocument.htm - "To create a document, create a new version via the ContentVersion object without setting the ContentDocumentId. This automatically creates a parent document record.")
• Insert ContentVersion record
• DEBUG: Confirm ContentVersion.ContentDocumentId is set (Here is where the problem is because it is NULL; however if I SOQL query ContentVersion after the Apex completes, ContentVersion.ContentDocumentId is set and the corresponding ContentDocument record exists)
• Create ContentDocumentLink record (commented out because it fails because ContentDocumentLink.ContentDocumentId is a required field)
• Insert ContentDocumentLink record (commented out because it fails because ContentDocumentLink.ContentDocumentId is a required field)

public class FileController {
    
    @AuraEnabled
    public static Id saveTheFile(Id parentId, String fileName, String base64Data, String contentType, Id contentDocumentId) { 
        base64Data = EncodingUtil.urlDecode(base64Data, 'UTF-8');
        
        ContentVersion cv = new ContentVersion();
        cv.ContentLocation = 'S';
        cv.ContentDocumentId = contentDocumentId;
        cv.VersionData = EncodingUtil.base64Decode(base64Data);
        cv.Title = fileName;
        cv.PathOnClient = filename;
        
        insert cv;
        
        //***This DEBUG statement must return an Id for the rest of the code to work***
        System.debug('contentDocumentId='+cv.ContentDocumentId);               
        
        //ContentDocumentLink cdl = new ContentDocumentLink();
        //cdl.ContentDocumentId = cv.ContentDocumentId;
        //cdl.LinkedEntityId = parentId;
        //cdl.ShareType = 'I';
        
        //insert cdl;
        
        return cv.Id;
    }

    @AuraEnabled
    public static Id saveTheFile(Id parentId, String fileName, String base64Data, String contentType) {         
        return saveTheFile(parentId, fileName, base64Data, contentType, NULL);
    }
}