In bundler/bundler & rails/rails project, there are some *.tt files (Go to the project page, click t then type .tt). Inside this kind of file, it could use ERB. Example of a .tt file (from bundler):
source 'https://rubygems.org'
# Specify your gem's dependencies in <%=config[:name]%>.gemspec
gemspec
What are these .tt files? Where are they from?
They're Thor template files.
The .tt could be any file extension you want. It is just because they pick .tt as file extension. See Thor::Actions#template to learn more.