Checkout Fields change

Hello and good day dear Woocommerce specialists.

I would like to change the checkout page, but the solution (found on the net) works with

add_filter( ‘woocommerce_checkout_fields’ , ‘custom_override_checkout_fields’);

only works for some fields, for the others the changes are only displayed briefly and then the “old” data is displayed.

Now I would like to change the data “directly”, but where are these to be found, in the depths of Woocommerce

LG

Lynn