Company building web app – better to duplicate content from existing knowledge base-like website, or link to the pages?

Hello everyone, first post here.

I am a BA working in a public service organisation of 10,000 people. The organisation has had a plain website since the beginning, and it more of a library/ public-facing knowledge base than anything.

Since about 2 years ago they have had a sparkly new web-app that is much cleaner, more focused, less words, mobile responsive, and you can actually do things in it like apply for things or update your profile. It's all very good compared to when they just had the old website!

HOWEVER, there is an issue that all departments/business owners are facing in that how do we deal with content?

• Do we copy and paste from the website into this new app? (making the purpose of the website vs the app more obscure) or
• Put links in the app to the website page that has the content? (making the user experience choppy and potentially confusing for newcomers "which website am I on now, is this still where I'm meant to be?" etc)

What is best practice?
What would you suggest?

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Cheers!