how I add 30% discounts on all products without editing one by one product.
from the discount option, it’s only applicable after adding to the cart. but we want discount shown on pages
Use csv export/import and compare at price fields
https://help.shopify.com/en/manual/products/import-export/using-csv
If your theme supports it to just have messaging about the discounts use announcement bar sections in the header, and in the product pages use text-blocks near the pricing.
Otherwise to change the price displays themselves you need a theme customization to hardcode the discount math with some logic to turn it on/off.
If you need this customization then contact me by mail for services.
ALWAYS please provide context, examples: store url, theme name, post url(s) , or any further detail.
Contact info in signature.
Good Hunting.