How to convert rails app to test files?

I like to create rspec test from working rails 5 app. It' could be template to work on.

For example: -scaffold will create test files template which is nice. -Or simplecov to help increase test ratio (if I understand correctly) -So if any tools that could run through each line of controller method and re-create each expected/result put in the test file.

Are there any gem or solution ?

Why ask this question?. I understand this is not a purpose of test. However test is to save time in future and now as well. Many rails app don't have test, and to go back each line of running code will cost again, assuming the app is good at this stage. If we can have all test at this point and use it to control / run for future development that would be good