Systemd adding the ability to boot directly into disk image downloaded via HTTP

Systemd also has a newer feature named sysupdate which allows systemd to manage the host operating system. It has some really nice features around integrity and trusted boot.

https://man.archlinux.org/man/systemd-sysupdate.8.en

Here is a discussion about why they chose it for building an appliance like operating system for GNOME:

https://discourse.gnome.org/t/why-did-gnome-os-choose-system...

Alternatively, there is also bootable containers:

https://docs.fedoraproject.org/en-US/bootc/getting-started/