What is the Chip-on-board ic used in calculator, how to obtain one and how to use it?
@Ahmed, if you want make a calculator, of course is just for fun, right? 

If you were NEEDing a calculator, you would buy a chinese one VERY cheap, and it would be better than anything you could build yourself. 

But, as a hobby project, it can be really fun build one for yourself. 

The starting point SHOULD NOT BE a commercial product. Forget about it. The chip for this calculator isnt available to be sold retail, and even if you can get some samples, you wouldn't be able to apply this chip to a circuit, because it requires expensive industrial machinery.

I think that the approach should be different.

An easier form would be to buy a complet kit, like this or this. If the links don't works, try search for KIT DIY CALCULATOR on Aliexpress or Banggood.

Even if you don't want to do it so easy, I think that kits like these are a good starting point to you decide what you want to do by yourself and what you'll buy for your project. You should think about the hardware and the software.

you could just use an Arduino, attaching a keyboard and a 16x2 display to it...

you could buy a simple keyboard to attach to Arduino (and there libraries to Arduino work with those keyboards), or could build your own with your own PCB and switches...

You could use a LCD display, or 7-segments led displays, and the hardware and software shall be able to use them.

You could chose a microcontroler and write its software yourself, and load it in the microcontroler, even you could need a chip programmer;you should learn a lot about software, because the calculator use numbers of many digits, sometimes more than the microcontrolers registers use.

you can use the PCB from a kit, you can build your PCB from an online project, or even, you can project and build your own PCB... or have your PCB project made in a factory...

I'll stop here, since you started only asking how to get the calculators chips. I fell that this question doesn't exactly address what you were needing, since the correct approach to build a calculator as hobby project is not try to reproduce or copy a commercial product. Good luck!