System takes too much space

OK. There is NO way to selectively clean System Data. The suggestion for closing Safari Tabs for some works, but not for everyone, depending on how many open tabs they have. And it's NOT bad System Data Management. Your phone is a computer, you just happen to be able to make phone calls with. There are many things your phone needs to have at the ready for it to work.

There is one sure way to reduce System Data and that is to backup your phone, then restore it from backup. This link explains how to do that --> Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

You will see System Data reduced after you restore the phone. But over time it will continue to grow and you can't stop it. Hence why I suggest that in today's world, a 64 GB iPhone simply isn't enough anymore.

Your system data contains a lot of necessary things for your phone to run. It contains your cache, system fonts and various temporary and system-related files necessary for the device's operation and performance. 20 GB isn't really out of line. For example, if you use Safari a lot and you have many webpages open in Safari, closing all the pages, will reduce System Storage.

You can backup your iPhone, then restore it from backup, which will reduce System data, but over time it will naturally grow again.

This is probably not what you want to hear, but 64 GB of total storage as you have on your iPhone 11, just isn't enough anymore. Apple only sells iPhones now with a starting storage level of 256 GB. Maybe it's time for you to consider upgrading to an iPhone with more memory.