How to clean my iPhone 12 Pro Max myself without downloading another app

Do you mean to free up iPhone device storage?

If so, see the following Apple articles:

If you need more space for an update on your iPhone, iPad, or iPod touch

If you need more space for an update on your iPhone, iPad, or iPod touch - Apple Support

How to check the storage on your iPhone, iPad and iPod touch

How to check the storage on your iPhone, iPad and iPod touch – Apple Support (UK)

Clear Other storage on your iPhone or iPad

Clear Other storage on your iPhone or iPad - Apple Support

See also -> What's the difference between device storage and iCloud storage?

What's the difference between device storage and iCloud storage? - Apple Support

sabrinafromgadsden wrote:

How to clean my iPhone 12 ProMax without adding another app?

What do you mean by cleaning and downloading another app.

There is no app that will clean your phone.