This article explains the limitation where folders within content libraries are not visible in the Salesforce Mobile App(Salesforce for iOS), and files appear directly under the library. It also mentions workarounds using file naming.
Due to current product behavior, the Salesforce Mobile App(Salesforce for iOS) can only reference one level of folders (the libraries themselves).
Therefore, the folder structure within libraries that is visible on a desktop environment is not displayed on the mobile app. Instead, files located within these folders appear flat, directly under the library.
There is currently no feature available to display the folder hierarchy as requested. Please consider the following workaround:
Use file naming conventions to include identifiers that simulate folders (e.g., "[Proposal]_CompanyA.pdf").
Note: Sorting files within libraries is not supported in the Salesforce Mobile App.
Note: Libraries are not available in the Salesforce mobile app for Android.
Idea Exchange: Add libraries to files in SalesForce1 mobile application
Create Folders in Libraries in Lightning Experience
Note: When viewing your libraries in Salesforce Classic or in mobile, you can see the folder structure that you created in Lightning Experience. Only one level of folders is viewable in mobile.
Data Access and Views: What’s Different or Not Available in the Salesforce Mobile App
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