How do I really delete cache to get more GB on my iPhone 7 Plus?

K ~ Learn how to delete your history, cookies, and cache in Settings:

Clear the history and cookies from Safari on your iPhone...

Also turn your iPhone off & then on again:

Turn iPhone on or off - Apple Support

If your iPhone 7 Plus is slow or glitching, does it still have its original battery?

And what's the Maximum Capacity % here?...

Settings > Battery > Battery Health

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1.

Try to free up iPhone device storage see this Apple articles below:

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

2.

What's the difference between device storage and iCloud storage?

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

See this Apple article: Upgrade to iCloud+ -> Upgrade to iCloud+ - Apple Support

See also: iCloud+ plans and pricing -> iCloud+ plans and pricing - Apple Support

Manage your iCloud storage

Manage your iCloud storage - Apple Support