Shelter uses the Work Profile. With android 15 you can enable Private Space, which is about the same, for a third instance of an app. Work profile requires an app that manages it like Shelter, private space just works.
AfaIk, Shelter is using the built in work profile of Android, as not every UI/ vendor has made it available to the users. (There is no visible button or option in the settings menu.)
E.g. Samsung smartphones have a "work mode", but the last time I've used it (It may be different in current models), it only allowed for second accounts of selected apps like WhatsApp and hence, was a crippled implentation of the Android feature. My Android 11 Samsung tablet has complete multi user support, not only "work mode".
If you are required to install something like InTune by your company, that's what controls what apps you can install in Work mode. It's a good trade off because it enforces good separation for them - clipboard is blocked between profiles, data too - but also for me. One little toggle and those apps and the profile are disabled entirely until I care again tomorrow.