You need to be well familiar with cloud computing as well as REST services, API Gateway, DynamoDB.
Set up AWS a cloud account, allow users to sign up with Cognito from mobile or PC devices
Built a database for text strings that will be send by the mobile App or the PC and organise them in folders and categories as indicated by the sending device.
Add a tags to the string that contain necessary auxiliary information as :
who created it and when / which folder and categorie/s it is a member of / if it is approved by the administrator for public download .....
Call Amazon Polly and convert and store the strings to audio files.
All logged-in users should be able to download converted audio files if tagged as“public” by the creator
This is only the first step in an ongoing project - which could give you lots of follow-up work - Mobile App and PC program is done by other programmers.