Which type of power banks are better for iPhone 11?

Practically anything will work.

It just needs a USB-A or USB-C port.

You should also look for one marketed with fast charging support or USB Power Delivery.

A guide is here: Fast charge your iPhone – Apple Support (UK)

Apple do also sell this sleek case with a built in battery for the iPhone 11

It should work perfectly fine with your iPhone 11.

This power bank seems to have 18W fast charging, so should be able to charge your iPhone 11 to 50% charge in 30 minutes as well.