How do I completely remove work phone data while retaining it on my personal iPhone?

I have a work phone and a personal phone, which have been linked and used collectively (yes I realise my mistake!). I now have to hand back the work phone, and would like to ensure that all data is completely removed from that phone, while retained on my personal phone (contacts, texts, notes, photos etc).

Please advise on the best way to do this - thanks so much.

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Original Title: How to