Debian trixie source.list (with unstable and experimental added) and corresponding apt pin configuration.

 This is the /etc/apt/source.list -- 

deb http://deb.debian.org/debian/ testing main contrib non-free non-free-firmware
deb-src http://deb.debian.org/debian/ testing main contrib non-free non-free-firmware
deb http://deb.debian.org/debian/ trixie main contrib non-free non-free-firmware
deb-src http://deb.debian.org/debian/ trixie main contrib non-free non-free-firmware
deb http://deb.debian.org/debian/ trixie-updates main contrib non-free non-free-firmware
deb-src http://deb.debian.org/debian/ trixie-updates main contrib non-free non-free-firmware

deb http://deb.debian.org/debian/ trixie-backports main contrib non-free non-free-firmware
deb-src http://deb.debian.org/debian/ trixie-backports main contrib non-free non-free-firmware

deb http://deb.debian.org/debian/ trixie-proposed-updates main contrib non-free non-free-firmware
deb-src http://deb.debian.org/debian/ trixie-proposed-updates main contrib non-free non-free-firmware

# ensure there is no testing-security and remove these
deb http://security.debian.org/debian-security testing-security updates/main updates/contrib updates/non-free updates/non-free-firmware
deb-src http://security.debian.org/debian-security testing-security updates/main updates/contrib updates/non-free updates/non-free-firmware
deb http://security.debian.org/debian-security trixie-security updates/main updates/contrib updates/non-free updates/non-free-firmware
deb-src http://security.debian.org/debian-security trixie-security updates/main updates/contrib updates/non-free updates/non-free-firmware

deb http://deb.debian.org/debian/ unstable main contrib non-free non-free-firmware
deb-src http://deb.debian.org/debian/ unstable main contrib non-free non-free-firmware

deb http://deb.debian.org/debian/ sid main contrib non-free non-free-firmware
deb-src http://deb.debian.org/debian/ sid main contrib non-free non-free-firmware

deb http://www.deb-multimedia.org/ trixie main non-free
deb-src http://www.deb-multimedia.org/ trixie main non-free

# not available now, but will be later
deb http://www.deb-multimedia.org/ trixie-backports main non-free
deb-src http://www.deb-multimedia.org/ trixie-backports main non-free

deb http://www.deb-multimedia.org/ testing main non-free
deb-src http://www.deb-multimedia.org/ testing main non-free

deb http://www.deb-multimedia.org/ unstable main non-free
deb-src http://www.deb-multimedia.org/ unstable main non-free

deb http://www.deb-multimedia.org/ experimental main non-free
deb-src http://www.deb-multimedia.org/ experimental main non-free

This is the corresponding pin configuration (you may place this in /etc/apt/preferences.d/custom)

Package: *
Pin: release n=trixie-security
Pin-Priority: 996

Package: *
Pin: release n=trixie-updates
Pin-Priority: 995

Package: *
Pin: release n=trixie
Pin-Priority: 991

Package: *
Pin: release n=trixie-proposed-updates
Pin-Priority: 990

Package: *
Pin: release n=trixie-backports
Pin-Priority: 550

Package: *
Pin: release n=trixie,o=Unofficial Multimedia Packages
Pin-Priority: 600

Package: *
Pin: release a=testing
Pin-Priority: 140

Package: *
Pin: release a=unsable
Pin-Priority: 130

Package: *
Pin: release a=experimental
Pin-Priority: 120