Q.plants are dying…why?
Recently, I have bought some plants of chilis and strawberries from nursery and transplanted. Big pot contains mixed soil (cocopeat, soil, vermicompost) in ratio 1:1:1, but it seems after transplanting all the plants are dying slowly…that is, day by day. I tried to plant other plants in it but the same happens. All the leaves first dried slowly and then the fruits also drying. What to do to stop this dying process? Please help.

Make sure the plants are watered carefully, the young roots need even moisture but should not be sitting in soggy soil.
If the container is quite large it may be holding to much water for the young plants.
You may have more luck using two different containers for these garden varieties.
Give them plenty of sunlight--5 plus hours.
Here are some links to refresh you on the care requirements.
https://www.gardeningknowhow.com/edible/fruits/strawberry/container-grown-strawberries.htm
https://www.gardeningknowhow.com/edible/vegetables/pepper/growing-peppers-container.htm