Apps will need to be built to work with the Shopify platform as apps found in the Play Store or Apple App store won't directly connect to your store out of the box. More information on how to create a Shopify app can be found here. Once the app has been designed to work with Shopify, you can add the app to your store as a "private app" - Private apps are not published to the app store but can work with your store specifically. If you'd like to have your app published to the Shopify App store then you'll need to ensure it meets all of the following requirements, then be submitted for review.
If you're not the app developer of the app you wish to use, then you may want to reach out to the app developer directly to see if you can hire them to make a Shopify app for your store. You can also hire a Shopify Expert to create an app for you. Lastly, you could try to find a similar app within the app store to meet your needs.
Trevor | Community Moderator @ Shopify - Was my reply helpful? Click Like to let me know! - Was your question answered? Mark it as an Accepted Solution - To learn more visit the Shopify Help Center or the Shopify Blog