no problem, maybe i'm just too stupid to understand

i have nem in my wallet. I connect my trezor to my wallet.
all the xem that i have inside my .wlt file right DON'T go automatically into my trezor and i don't have to make a transaction to trasfer them to my trezor

The Trezor is a completely separate new wallet. Right now there is nothing in it.
Note the address. Send everything in your existing wallet to it. Obviously do a test transaction first to make sure you have the right address.
Make sure you store the Trezor seed in multiple forms in multiple locations. That's your only backup.