How to Unit Test a Ruby on Rails Application

My company recently received a new contract in which the client's technologies are Ruby and Ruby on Rails. I've finished learning those, but am really struggling to learn how to do Unit Testing. Since Unit Testing (and then other testing after that) are the primary goals for the first portion of the project, it's crucial that I figure this out.

In all of my research and googling, I've come across technologies such as Rspec, Faker, FactoryBot, Capybara, Launchy, MiniTest, etc. However, I'm not finding anything super helpful with any of it.

Please advise the best gems to use and links to learn, in depth, how to use those gems.

Thanks in advance!