On GrapheneOS, it's like a container, or a virtual phone. Apps in different profiles (and you can install the same app in more than one profile) can't see each other and theoretically can't even tell they're running on the same phone (although I'm sure there are leaks like IP address)

You can make different profiles. They can have different unlock methods and can have different apps installed. If you have one app installed in both it's shared.

They were designed so multiple people could use one device.

Some people use them to separate identities or contain apps they view as bad. I'm not sure if the efficacy of this.

Grapheneos improves them significantly https://grapheneos.org/features#improved-user-profiles

It is a user thing, you can set up multiple profiles and install apps into each of them. These profiles are isolated from each other. I think they started out as a way of separating private and work apps/data, but you can have many of them. See e.g.:

https://grapheneos.org/features#improved-user-profiles