what happens to the data on my old device after I transfer data to my new device?

They have been copied & pasted to your new device, if you used one of the methods listed here:--> Transfer data from your previous iOS or iPadOS device to your new iPhone, iPad, or iPod touch - Apple Support, but they are still on your old device too.

If you completed the procedure successfully and you want to cancel your data from the old device, use these steps:--> What to do before you sell, give away, or trade in your iPhone, iPad, or iPod touch - Apple Support

Regards

when i did the transfer from my old device to the new one i lost all my password i had store on it and keychain wont turn off either any advice for this please?