As independent Linux stable distribution (without extra add-ons) PureOS isn’t able to do everything (neither every Ubuntu is able to play multimedia content out-of-the-box, I guess) but same rules, as linked within post under, when extra main package(s) needed, should confidently apply as for other Debian based distributions, please read and understand thoroughly (why not) entire thing (merge those before installing needed codecs, etc.) even before you start, on your own: “To install something from backports run one of: (sudo) apt install <package>/bullseye-backports …”