Great answer from Carlos. It helped me a lot.

Although, my /etc/apt/sources.list file is empty, only importing files from /etc/apt/sources.list.d, then I had to edit /etc/apt/sources.list.d/debian.sources instead, to include the contrib and non-free components. It looks now like this:

Types: deb deb-src
URIs: mirror+file:///etc/apt/mirrors/debian.list
Suites: bookworm bookworm-updates bookworm-backports
Components: main contrib non-free
Signed-By: /usr/share/keyrings/debian-archive-keyring.gpg

Types: deb deb-src
URIs: mirror+file:///etc/apt/mirrors/debian-security.list
Suites: bookworm-security
Components: main contrib non-free
Signed-By: /usr/share/keyrings/debian-archive-keyring.gpg

Then, I run:

apt update
apt install snmp-mibs-downloader

Everything is fine now.