KaiOS is just the new moniker for Firefox OS. It has NOTHING to do with Android aside from sharing a sort of Material-design aesthetic. More recent versions offer a few Google apps (Assistant, Maps, and YouTube) but I don't think it's possible to run Play Store or standard APKs.
I had a KaiOS flip (the Go Flip) for a couple of months before the experience made me pull my hair out. The OS is very unstable, full of ads, and randomly freaks out. I had to restart my Flip a few times a day because the keypad just stopped working (like the driver that it used crashed). It was slow, T9 was useless, and sometimes SMS messages got weird timestamps making newer messages show behind old messages and forget group texts if that's your thing. The phone was so cheap as to seem to be a crippled on purpose device meant to make folks run to a smartphone so the OEM could use that to say 'see? SEE? flip phones are sooo 2005!'.
Now, today, I use a much better flip phone, running a pretty bare bones Android 5.1.1 build (AOSP). It is a total breath of fresh air. No background data usage, no Google, and full on privacy, and even a week of battery life. This phone feels far more premium than any KaiOS device. It's another example of 'you get what you pay for'