Hi Orlando,
On a running system it is in /boot/firmware. On the SD card it's in the boot partition. However, you should use the dietpi-config tool to set your sound card. Log in as root (or type 'sudo su'). Then:
dietpi-config
Select Audio Options
Select Sound Card
Then scroll down and select your hifiberry card.
If you look at the config.sys file it will look a bit different than the RPiOS one. Try the dietpi-config method first.
Randy