https://www.trip.com is 90% Rails I'd say.
Everything you see on the surface is Rails.
As you grow, you do less of "belongs_to" and "has_many" and you do way more Ruby and way more Glue code that will work with all the models in the system and generate something for the user.
Scaling is a big issue, you constantly work around "best practices" that don't scale. (active-model-serialization for example) in favor of more fine-grained control over your models and data flow.
These days, the work is about 45-45-10, 45% you do APIs, 45% you do JS/React/etc... and 10% you do "Rails" (likely even less).
When the project grows, the monolithic nature of it starts to be a very big pain in the ass. We are moving more and more into services that do one thing well and not automatically include everything into the project.
Hope this answers your question
Tips:
Forget Rails, Learn Ruby, HTTP, how things flow from the request to the response. Adopt universal best practices.