Hi,
If this jar is for S3 datastore, this should be placed by your IaC (infrastructure as Code) or configuration management system as this is needed for the base vanilla AEM instance to work. You should place it inside /install/15/ folder along with other jars like oak-blob-cloud. Only the configurations needs to be present in /install/ folder.
If the jar is for your application, then it can be placed in /apps/<app-name>/install folder which will be picked up by AEM.
If this is for any functionality in your application, you should add its a maven dependency in your POM.xml