How to add product description section?

[sengho66]

https://demo.gatsbystorefront.com/product/matilda-linen-dress

On this product's page, there is a size guide and care for linen, where to add those details on shopify product?

Also, I notice there are two descriptions, one is right below the price tag and the other is "Item Details".

The one right below the price tag, where do you add that in shopify product?

Thanks.