I grew a few apple seeds 4 weeks earlier in a refrigerator and the seeds have now germinated. Please see the picture. Can you please suggest when should I plant them in a pot. For now I have kept them in the fridge under 4 degrees Celsius, with a little water.
Get them planted in some great seed starting soil. It will be potting soil, very fine and sterilized. No larger than a two inch diameter pot. And do this NOW. Your paper towel is a bit too wet. Wring it out and put those seeds that haven't germinated back. Put in a zip lock. Check every day. Once you see the seed casing split and/or a white root coming out, transplant into a tiny pot with potting soil. Keep moist, not soaked.
Get larger pots prepared: The next size should be 4" in diameter with sterilized potting soil. Transplant and up pot size when the roots are visible from the hole in the bottom. Add a tiny bit of fertilizer when you get to the second up potting into the 4" pots, balanced fertilizer NPK all should be equal numbers.
I have to disagree with Ecnerwal. Getting trees started in a nursery always starts in pots and potting soil. Sowing directly into the soil is not how nurseries produce trees. In my experience anyway and in the zones I have gardened.
But his advice on the ability to get a true variety is right on. But hey, why not? You are growing and learning and shoot, this is how one becomes a plant person.