This is incorrect as Asahi Linux can also be installed from macOS Recovery, which is the Apple UEFI equivalent.
I don’t want to continue running the old Catalina system.
Is there a way to install Linux on that bare metal? What’s the best approach - direct install, or use something like QEMU underneath?
You just flash the installer of your favorite distro (don't use ventoy) to a USB and install as usual.
If you need to install Asahi, you should install it as per the instructions in a dual-boot setup, then delete the macOS partition and resize afterwards.
What I’m most interested in is converting my 2012 Air to Linux for development purposes. The hardware is good! With 4GB, it’s really too small to run recent MacOS, and it will be unsupported soon, but it should be fine for i3/Linux.