Lost in translation, I think. I mean you reuse the connector and controller and sensor from the original battery. Then you can connect any voltage 3.7-4.2 Volts to instead of Li-On cells.

There is of course the issue that a phone uses momentarily high current when transmitting. So there needs to be beefy capacitor too.

I have done this many times, of course. Especially on Ebooks, for which you cannot find replacement superthin batteries. You use original connector and any small lithiums in parallel. Discarded e-cigarettes are good source for small lithiums.