How can I back up all my data in my new iPhone when I do trade-in

Back up all data you wish to preserve frequently.

How to back up your iPhone, iPad, and iPod touch - Apple Support

Then thoroughly cleanse your old iPhone using the article linked above your post.

Then transfer the data to your new iPhone.

Transfer data from your previous iOS or iPadOS device to your new iPhone or iPad - Apple Support

Please follow the steps on these Apple articles, basically backup and erase your old one and then restore the backup into the new one. Enjoy...

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

See the following Apple Support article for information on backing up an iPhone: How to back up your iPhone, iPad, and iPod touch - Apple Support

To migrate data to a new phone, see: Transfer data from your previous iOS or iPadOS device to your new iPhone or iPad - Apple Support

Note that the Quick Start option doesn't require a backup.

Then, to erase the old phone, see: What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support