Authenticating Request from shopify store front to my app backend

Hello guys,

I am new to shopify eco-system,

I am trying to build something where for rendering a block on storefront I have to call my app backend
Which in turn will call shopify graphql for few other details,

I don’t know and unable to figure out how to authenticate
Storefront <—> App backend
App backend <—> shopify grqphql

I m very confused in this flow,
also orthogonally,
Earlier I was hoping that liquid might work to change the said block/section of the website part but their lack of json parsing and fetching external data is too disappointing.

Thanks in advance.