Note : In this question "Tag" means Tag or a Custom Taxonomy (Assume that it is topic).
It is hard to explain my question with words. So I will explain it using following example.
When user visit example.com/tag/google/ or example.com/topic/google/ should be out all posts which has 'google' word in title.
I am going to use
pre_get_posts action to change the loop. But I have no any idea how to modify the loop. I could not find any thing in WP Query documentation also.