如何在没有Amazon Cognito的情况下使用AWS S3？

I am using a Node.js REST API to authenticate my users. Once they are authenticated I allow them to post photos with text. My plan is to store the text and the URL to the photo in the database. That way when they go to the feed of posts, my app will query the database to get the text and URL's and then use all of the URL's to get the images from S3 directly. Is this the correct way to do it and if so how come I can not do so without using cognito. I am trying to cut costs and it seems like cognito would be useless since I already add authentication with my API.I am using a Node.js REST API to authenticate m