Temp and Junk files

When you Offload an app, data from the app is kept locally on your iPhone, so if you choose to install the app again, you won't lose any data. The advantage here is that you save the space that the app takes up, but not its data. If you delete the app, both the app and data are deleted - unless it is stored in iCloud.

iOS itself cleans up temp and cache files when there is a need for it. There is no button to clean up with, or a "1 Click Cleaner" app in the App Store.

If you have very little space left, you can try to download a large file - search for a test files on Google. When iOS finds out that you are trying to make room for a file of e.g 5GB?, then iOS will start removing cahce files you can live without. Remember to be on WiFi if you choose to try this method. Unless you have free data in your plan. Finally, you can also simply restart your iPhone. When restarting, it is also cleaned up a bit, just like when you do a software update.

The completely impractical advice is to remove the apps, images, music, messages that you no longer need.