How to make milk kefir

I’ll explain something to you about how I make kefir.
-I put three spoons of semi-solid kefir from other times with 1/2 liter approx. milk. I move it.
– I leave it to rest for 12-24 hours uncovered until it sets. It usually curdles 3/4 parts, sometimes half, and the rest that remains at the bottom remains in serum. I move it again and put it in the fridge, covered. I never strain it.
I consume it as is, moving it previously. It’s strong but it hooks me. When I want to do more, I repeat the initial operation.
This are my questions:
Is it normal that when I let it rest for 12 – 24 hours, the bottom part remains serum? or the whole pot should curdle.
Is it correct to use only curd kefir to reproduce it? I have never used kefir nodules to make more. Thanks for your information.