how do I download all texts from my iphone with a person ?

There is no function or facility to do that natively on an iPhone unfortunately, outside of taking screen shots of every screen you want to keep.

You can offer feedback to Apple by clicking the following link.

click here ➜ Feedback - Messages for iOS and iPadOS - Apple

Sosachse Said:

"How can I download full text threads / messages with someone from my iphone ?"

-------

Use my User Tip: How to Save or Print a Complete iPhone Messages Thread - User Tip

You cannot do that with the phone, but there are 3rd party apps for Mac and Windows that can create neatly formatted PDFs of conversations. Two well liked ones are TouchCopy and iMazing.