Different person, but I'll try to explain some of what I know.

Traditional Linux:

• read/write root and usr
• only one version of a program can exist*
• packages are available immediately after install
• packages are imperative (you tell it what to do, it does that)
• files swapped in place (can lead to issues like kernel modules missing or Firefox not opening new tabs until restart)

*you might have python3.8 and python3.9, but those must be created as different packages using different paths in /usr

NixOS, Guix:

• declarative package management (basically config file and exactly these packages are installed)
• usr and parts of root read-only (afaik)
• packages symlinked to usr
• multiple versions of packages kept locally (though not all active necessarily)
• will keep using old package until restart/reboot, therefore not breaking on updates. New instances of a program can use the new package
• easy to roll back due to multiple versions kept

Immutable OS (haven't seen one mentioned by OP, but it's a category):

• often imperative package management
• using snapshots or multiple root partitions for easy rollbacks
• read-only root and usr
• packages might only be available after a reboot (depends on implementation and if system packages or something else like Flatpak, which doesn't need a reboot, are used)

SerpentOS:

• experimental distro (ie stuff might change)
• imperative package manager
• packages installed to separate tree, but swapped live. Basically A/B root of an Immutable system that doesn't require a reboot (according to the explaination in the latest blog post)

Not sure why ClearLinux is on that list of special distros and I don't know half of the rest so yeah. Hope this explains some of it?