'Related keyword' suggestion system (30$)

'Related keyword' suggestion system (30$) Hello! Thank you for having interest in this topic. You know, when you navigate into search engines such as [login to view URL] or [login to view URL] and search a keyword. They provide related keywords list. I wonder logic about this. I see most of the search engines have functions like this. Please see the specific questions below. Question 1. How could they calculate relatency between keywords when a user searches it? Question 2. Which and which materials do we need to have such 'related keyword' showing system? Question 3. Do you think an individual is able to construct system like the search engines I mentioned above? Even if I say only calculating relatency between keywords part? Thank you :) waiting for your answer. This could be root of our future project perhaps.