around android 10's release, google introduced a neat new feature called dynamic partitions. a year later, they would go on to further iterate on it, introducing dynamic system updates,
DSUs are basically second OSes. or at least, the user-facing implementation is. android stores a system.img and userdata.img file in the FS at /data/gsi/dsu, which android then proceeds to load as the userdata and system partitions on boot when the DSU reboot is called.
however, it has limitations. on anything other than a pixel, the bootloader must be unlocked before loading even the default google-compiled GSIs offered in developer options. on pixels, the official GSI builds can be loaded while locked, but that's about it. custom ROMs always need an unlocked bootloader, just as they would when you actually try to flash them.
the dev options menu for installing a DSU is, for lack of better words, utter complete shit. the data partition is made the smallest it can possibly be, making it essentially useless once booted. luckily, there's several ways to load a DSU, including through a command-line interface and a handy open source app designed to let you set up a DSU right on your device, given you're at least rooted.
actually using the DSU
with the explanation out of the way, this is their practical functionality. expect no features to work fine out of the box. GSIs are iffy and that's just their nature. they're founded on duct tape, futile prayers and the hope for a nice OEM that gives a half-decent vendor partition. this system is called treble. by now, most manufacturers support it, given it's a GMS certification requirement and has been since at least 2018.
some manufacturers are known for doing a pretty decent job on treble compatibility. google comes to mind, and oneplus isn't that bad. my single (now dead) motorola phone was also quite fine on a GSI. it mostly depends OEM to OEM, and device to device. i like mystic GSI because they offer OEM skins as GSIs, encouraging my unhinged ass to make some cursed devices.
i recommend picking a mystic GSI image with the same chipset as the target device you intend to install it on. the hyperOS image works on both my oneplus 12 and razr 50 ultra (rip), though the op12 has gfx accel issues that make the OS laggy and glitchy. pixel images caused an EDL brick on my op12, so i guess don't do that. i recovered with a force reboot, though.
the oxygenos/coloros images seem to be genuinely fucked. i can't even install them on actual oneplus hardware. oneui images get to the boot screen, even play the boot animation...then they crash. they probably are mostly compatible with xiaomi devices, as i see the dev telegram group full of ximi pics.
is this viable for daily use? NO. i couldn't get radios working in even the google-compiled GSI with my oneplus 12. but it's a cool novelty feature. a solution to a problem nearly no one has, but a welcome one by the people who do have it.