https://wiki.ubuntu.com/WubiGuide
They've since dropped the idea but you can still cobble together something similar.
https://unix.stackexchange.com/questions/309900/deploy-linux...
That said this was back during win7, the cobbled together solution may be more stable..
The really nasty part is that even having separate drives won’t save you - I don’t know if Windows can install the bootloader to a different drive than the one holding the OS partition or if it’s some other bug but I absolutely remember a Windows install nuking the bootloader on other drives that were connected during the OS install.
It wasn’t a big deal pre-Windows 10 as you only install an OS once and disconnecting the drives is trivial, but nowadays with every update being an OS reinstall it’s a major problem if this issue still persists.
Every normal OS registers a boot option with the UEFI and puts its bootloader into a normal directory. It also pushes its own bootloader as the default in some cases, but that's just a single boot order change away after booting into the UEFI setup (or after messing with confusing and dangerous tools from inside Windows, which I wouldn't recommend).
The dual boot problem is only really insurmountable in BIOS+MBR mode (which should be considered long deprecated) and with buggy motherboards. Some people will set their system to BIOS+MBR mode for some weird reason, but with Windows 11 this mode should finally be dead already. UEFI has been an option since at least Windows 7 but it was disabled by default for a long time, so if you've upgraded to Windows 10 from a previous version, you'll most likely still be running this old and unstable config. I think this is one of the reasons new Linux users have so much trouble dual booting.
Modern motherboards detect different UEFI bootloaders installed on EFI partitions even if you swap around SSDs. Dual booting has come a very long way!
There's another problem. Motherboards with non-compliant UEFI specs. Exact details here may be flawed, but one of the most insidious dual boot issues I had, which prevented grub from loading was that it turned out, if you had a folder or file labeled windows or Microsoft in the EFI partition, it would always preferentially boot that one no matter what order you set to actually prefer. To fix it, I had to run the Linux EFI utilities for OS detection to set everything up, then go in and manually change everything with windows to some other name. This included modifying whatever the grub configuration file is that says to never modify manually. The biggest issue I had is that it took a LONG time to figure out that noncompliant UEFI specs on the motherboard was the problem. It was maddening.
https://bugs.launchpad.net/wubi/+bug/1471344
Removing wubi was a quick fix instead of patching it.
Wubi also had a dependency on grub4dos which had an idiosyncratic development and release style, to say the least. It was also the only component developed by non-English speakers, which made communication very hard. Later releases had some major regressions that were never sorted out, and I think everyone involved found it highly draining to deal with these issues without a more fundamentally correct way of tracking and preventing regressions.