Hi Ulises

login.mustache is the old login screen -- the new one is built in Vue.
This was changed in TL-36674 with the changelog entry:

TL-36674 Login screen refresh A new login screen is available with a new look and feel, and a prominently presented image to the side. The image can be configured inside theme settings. The old login screen is still available, but is deprecated in Totara 18. You can revert to it by setting `$CFG->enable_legacy_login = true;` in config.php.

You can switch to the old one by changing the config.php file as above or a better idea is to switch to the new one located at client/component/core_auth/src/pages/Login.vue and override in a theme by following the instructions here: https://totara.atlassian.net/wiki/spaces/DEV/pages/121187198/Creating+custom+themes#Overriding-component-styles-or-functionality

Regards