Thanks for the answer. Well understood.
Now the following question is:
Is better to have mini sites at key1.com, key2.com and key3.com (linking like you say to the main website) or to have subdirectories in the main site like www.domain.com/key1/ www.domain.com/key2/ www.domain.com/key3/
containing the same content that in the mini sites?
You don’t need to create new pages on your site if the homepage is targeting the same keywords. If it doesn’t then yes, the links should point to relevant pages that target those specific keywords.
Your answer is the first part of you question i.e., It as better to have mini sites at key1.com, key2.com and key3.com (linking like you say to the main website).
But do not have the same content in your subdirectories. it may result in duplicate content. You can have a back & forth link from your mini sites or you can use a read more
the concept of the keyword domain is that a good percentage of the searchers are too lazy to search for a product. Example - iphone. They usually do is type iphone and use ctrl+enter, making it go directly to “http://www.iphone.com”. So if you have the keyword domain you can tap into these traffics and moreover it is easier to rank keyword domain faster.