KaiOS creates the software that makes smart feature phones work, but we don’t build actual phones. Partnering with manufacturers enables us to make our software usable worldwide.
KaiOS provides manufacturers with a "blueprint" that they can modify. A manufacturer can choose to add a feature or leave it out, and to create their own store for apps in place of the KaiStore.
In the case of the JioPhone, Reliance Jio opted to implement the JioStore instead of the KaiStore.
A KaiOS device can only have one app store, and it isn’t possible to change the built-in app store on your phone.