How can I clean the cache on my iPhone XR?

it's pretty easy, check this out: Clear the history and cookies from Safari on your iPhone, iPad, or iPod touch - Apple Support

It allows you to clear Safari cache.

Unlike Safari, clearing cache for other apps depends on what the developer allows. Some apps have an option to clear cache, while others don’t provide this.

To check, go to Settings and scroll down the list until you see your installed apps. Tap each one to view its specific settings. You may see a Reset Cache on Next Launch or similar option.

There's no a universal "Clear Cache" tool in iOS, since the system removes it itself when it needs. You can clear the cache of Safari under Settings > Safari. Some other apps offer clearing cache inside them.