While others already pointed out the similarity to persistent LiveUSB, I would argue that this also feels a bit like Android desktop modes, like Samsung Dex.

yes, valid point, thank you for the correction

As jet points out, QEMU for actual hardware virtualisation.

There is one relevant thing, which is not exactly in the same category, but does somewhat similar thing:
containers
most popular example being Docker
https://en.wikipedia.org/wiki/Containerization_(computing)
containers don’t emulate whole hardware stack like virtual machines do, they just run the guest OS on top of host OS.
so because they don’t put resources towards emulating hardware, they are much more resource efficient.
so if your problem is “I’m running Fedora but I want to run something that for some reason runs just on Ubuntu”, then you could use containers for that.
containers are mostly used in headless environments (as in servers, no GUI), so running and displaying desktop Linux inside them is a bit tricky, but it can be done.

Lemmy is not like Reddit, you can edit the title even after the post was made.

Also, the latest release of Linux Mint is Xia:
https://www.linuxmint.com/download_all.php
https://en.wikipedia.org/wiki/Linux_Mint#Release_history

Listen, strange penguins biting people is no basis for a system of government.