Can a subpage rank better than a index page?

Can a subpage, lets say xxx.com/buyxxx.html rank better than someone's yyyy.com assuming that they both are marketed equally? Would google give high priority to a site that has several pages dedicated to a keyword like mns or a single optimized page with backlinks to it can rank equally well?

Yes, it's possible.

Yes.

A homepage is simply just another page in SE's eyes.

One of my site, its homepage ranks on page 4 while one of its page was ranking #1.

The key difference with home pages is that they're much easier to rank in the first 6-12 months of a domain's life. So if you were comparing 2 new domains then given all other factors remaining equal (which is never the real case of course) the homepage will rank quicker.

Over time, the home page normally carries the most juice simply because all pages usually have a link to it.