IOS 15 does not support newer apps. How do you get older version of the app for my iPhone 7

Unless you have downloaded them previously, you won't be able to.

You can get a compatible version if you have previously downloaded the apps, by going in to your Purchase history and attempting to re-download the app from there.

click here ➜ Redownload apps and games from the App Store - Apple Support

If the app is no longer on the App Store the developer has pulled the app from the store. It’s the developers option whether to offer different versions of an app and/or how long to support older versions of the operating system. You can always try talking to the developer. If course, it’s also possible that the hardware has updated enough, that the older version can no longer be supported. It can even be a case of safety and security, where the older app can’t be made safe from the more advanced security threats.