I mean how is ssr possible

With random backend framework basically

while using nextjs as fullstack solution, you fetch data in your server components

but if you're using custom backend, you make api requests to your backend to get the data

Exactly. You use fetch to access any external data. On server-side, direct fetch (or any wrapper around fetch will do), on client side the same, and this is where you can use something like TanStack Query to access external data (although I'd stick mainly to server-side fetching as it will make it easier to cache and/or protect)