That's a great question!
Device manufacturers provide the hardware, which are tangible components and physical devices (phones) that are necessary for storing and executing (or running) the software. KaiOS is the software, which refers to a collection of programs, documentation, codes, instructions, etc. That’s why KaiOS cannot resolve hardware-related issues (eg. swelling battery, broken screen etc.).
KaiOS works with numerous device manufacturers, such as HMD/Nokia, Reliance Jio, Bullit, Alcatel/TCL, Doro and many others by providing software for their end products (feature phones). Device manufacturers can choose to change or even leave out some of KaiOS’ features, so it’s best to do your research to find out which phone best suits your needs.
To find out if the phone you're planning to buy or have bought has a particular feature you're looking for, or in case you have any hardware-related issues, please reach out to your device manufacturer.