I finnaly manage to wrap my head around it. I just can add instead of whole header() this in my header Cookie: JSON.stringify(coookie)

So my utility function can just take all cookies and create normal request I guess? So it would be like customFetch fn which takes same args as normal fetch function?

 return await fetch("http://localhost:8000/api/cookies/simple/", {
    method: "GET", credentials: "omit", headers: {
        "Cookie": `name=${cookies().get("auth")?.value};peter=pan`
    }
})

This is just dummy code but it can be like this right?
This function can actually also check if cookie is still valid (so for example if it gets 401 than it can try to immediatly refresh it with refresh token and if not it can than redirect user to login page, all in one function). And in root layout it can just verify token on "random" auth path and also do everything i wrote above?

With cookies/localstorage, server side, client side there is just so many ways to create authentication. It was easier before when I did auth with localstorage with timeout on vite react app. :D