many linux distributions still use the older method of getting things to appear on screen.
they rely on a program called x11, development on x11 has stopped in favour of a new system called wayland. x11 does not support running different displays at different refresh rates, wayland does.
but nvidia doesn't support wayland very well yet. you can use it, but it might be more prone to crashes when using an nvidia gpu. i still recommend trying it.
usually you will find a menu at one of the corners of the screen before logging in to your desktop. here you will usually find something like "desktop name (xorg)" and "desktop name (wayland)".
but some software hides the wayland option from nvidia users, it shouldn't be too difficult to find a guide on how to make this option appear if it is hidden though.
HDR support is still a work in progress. Afaik it's not part of any official standard for display technology on linux yet, but KDE Plasma 6 has experimental support and Valve is actively working on support.
KDE Plasma 6 is currently only available on distributions that push updates more frequently, without testing said updates thoroughly, like arch linux and some derivatives. the pop os developers have also promised to support HDR in their upcoming desktop environment called cosmic, which might still take a while to be released