Example to create salesforce workspace folders programmatically.

README.md

Reference

https://salesforce.stackexchange.com/questions/265708/querying-and-creating-folders-files-from-within-a-library

ManageContentFolder.cls

@testSetup
private static void createTestData() {
    ContentWorkspace testWorkspace = new ContentWorkspace(Name='TEST CLASS', DeveloperName='TEST_CLASS');
    insert testWorkspace;
}

@isTest
private static void testGetFilesList(){
    ContentWorkspace testWorkspace = [Select Id from ContentWorkspace where Name = 'TEST CLASS'];

    ContentFolder workspaceFolder = [Select Id from ContentFolder where Name =: testWorkspace.Id];

    ContentFolder testFolderRecord = new ContentFolder(Name='TEST FOLDER', ParentContentFolderId=workspaceFolder.Id);
    insert testFolderRecord;

    List<ContentVersion> contentVersions = new List<ContentVersion>();

    for(Integer i = 1; i <= 12; i++) {
        String fileExtension = '';
        Switch on i {
            when 1 {
                fileExtension='csv';
            }
            when 2 {
                fileExtension='jpg';
            }
            when 3 {
                fileExtension='gif';
            }
            when 4 {
                fileExtension='tiff';
            }
            when 5 {
                fileExtension='ppt';
            }
            when 6 {
                fileExtension='xls';
            }
            when 7 {
                fileExtension='xlsx';
            }
            when 8 {
                fileExtension='doc';
            }
            when 9 {
                fileExtension='docx';
            }
            when 10 {
                fileExtension='pdf';
            }
            when 11 {
                fileExtension='pptx';
            }
            when else {
                fileExtension='json';
            }
        }

        ContentVersion cv = new ContentVersion();
        cv.title = 'test content'+i;        
        cv.VersionData = Blob.valueOf('Unit Test Attachment Body'+i);
        cv.PathOnClient = 'test content'+i+'.'+fileExtension;
        contentVersions.add(cv);
    }

    insert contentVersions;

    List<ContentDocumentLink> documentLinks = new List<ContentDocumentLink>();
    List<ContentFolderMember> folderMembers = new List<ContentFolderMember>();

    for(ContentVersion cv : [Select Id, ContentDocumentId from ContentVersion]) {
        ContentDocumentLink cdl = new ContentDocumentLink();
        cdl.ContentDocumentId = cv.ContentDocumentId;
        cdl.LinkedEntityId = testWorkspace.Id;   
        cdl.Sharetype = 'I';
        documentLinks.add(cdl);
    }

    insert documentLinks;

    for(ContentFolderMember cfm : [Select Id, ChildRecordId, ParentContentFolderId
                                   from ContentFolderMember
                                   where ParentContentFolderId =: workspaceFolder.Id]) {
        cfm.ParentContentFolderId = testFolderRecord.Id;
        folderMembers.add(cfm);
    }

    update folderMembers;
}