I am a lamer in the knowledge of the Raspberry Pi platform, I bought myself a Raspberry Pi 4. I want to make it multibootable, but not with the help of BerryBoot or PINN, since I do not like their functionality, because, if I understand right, that in the first case, the OS images must be specially adjusted from https://berryboot.alexgoldcheidt.com/images/, and in the second case only those that are in the list. And _{I want to install several OSes and, ideally, to load them like on my PC using GRUB2 or similar way}. However, I do not know how to install GRUB2 on the micro SD correctly. The OSes I want to install are Rpi OS and Haos (Home Assistant OS, which neither BerryBoot nor Pinn is listed). There is no BIOS or UEFI on the Rpi4, I am looking for information on how it could be done.
_{There are several articles with useful information that anyway hard for my brain:}

• https://wiki.ubuntu.com/ARM/RaspberryPi#Booting_generic_arm64_ISO_images
• https://forums.raspberrypi.com/viewtopic.php?t=282912
• https://forums.raspberrypi.com/viewtopic.php?t=306511#p1833566
• https://raspberrypi.stackexchange.com/questions/99473/why-is-grub-incompatible-with-the-raspberry-pi-answer-its-not
• And something like that https://pete.akeo.ie/2019/07/installing-debian-arm64-on-raspberry-pi.html

^{My problem is that I do not understand the principle of installing multiple OSes on a Rpi, but I want to understand and, most importantly, use it as a universal one. For example, I installed Rpi OS on Home Assistant OS, but I came up with a new project on raspberry and wanted to remove HaOS without completely formatting the media and install another OS, like Ubuntu. Or Ubuntu and Windows 10 for it, which, of course, is suitable for ARM64 on whiich is Rpi4. For what? Because I want, haha}

Any ideas? Has anyone done this? Maybe it’s elementary for Linux users, but not for me as a Windows user. So I would glad for help, but I repeat, that I want understand universal principle of work with DIFFERENT OSes for ARM64 (Rpi4) architecture, not like “install and use to next full erasing of flash drive”. Maybe I put the question or information wrong somewhere, so correct me please

In order not to confuse you and myself, I will not say what I tried, especially that I did not succeed in anything serious. Better try to describe the general points of what I need to do, preferably directly for the narrow-minded. I think it would be very useful guide not only for me