So I think I started thinking to much and now I confused my self

For context I am using prisma and next-auth

I originally just has a file called
queries.ts
and i was doing stuff like

await db.posts.find({})

and then i wanted to add this query in the same file

const session = await auth();
await db.posts.find({
where: {
  userId: session.user.id
  }
})

but I couldnt do this because of something about headers and i wasnt really sure what the real issue is

so i created a new file called
actions.ts

and ran the same query

"use server"
const session = await auth();
await db.posts.find({
where: {
  userId: session.user.id
  }
})

and it worked

but now im confused on whats the difference between a server action and just the file itself, maybe prisma is confusing me, isnt prisma able to reach the server without "use server"

i guess my main question is when I should be using server actions versus not, and if queries.ts is not a server action what is it?