So what are the provisions to extend maternity? Say one takes 6 months and wants to extend it a further 1-2 years... what's the procedure?

You could opt for WFH (basis child care) too, of course after discussing with RMG/HR. The reason would come under Personal Exigencies.

And that Child care leave will be non payable and that leave you can take once in a lifetime during the TCS.

Please note that TCS give 3 types of long leaves.
Child care 2 years without Pay- For female
Parents Care 2 years without Pay- to everyone
Master Degree Leaves 2 years Without Pay- to everyone

So if you will take any of the above leaves, you are loosing days other 2 leaves.

Because TCS gives you maximum 2 years of leave during lifetime.

Now it's upto you how you are managing those 2 years in the form of above 3 types of leaves

You can apply LWP for 1 year on the basis of chil care

thats child care. I believe they have some policy around it, check with hr once