Do I have to pay anything for the Singlife savings plan and how does the 1% interest rate on the $90k works? Does the 1% interest rate combine with the 2% on the first $10k?

If you are referring to https://singlife.com/manage/ then nope, there are no fees but you need a minimum account balance ($100?) to earn interest.

The first 10k gets up to 2% p.a. and the remaining amount until 90k gets up to 1% p.a.

So if you have $30,000, $10,000 gets up to 2% p.a. and $20,000 gets up to 1% p.a. and the sum of these two interests is what you get.