How to integrate Postgres RLS into backend server

Question

I'm trying to understand backends more and using row level security seems to be great idea to simplify backend code and increase security. I struggle to understand how to make RLS work. Let's say i have the following policy

CREATE POLICY user_mod_account ON accounts USING (username = current_user)

How does it know who is 'current_user'. I've followed few tutorials for making backend servers and if i'm not mistaken the way auth works is that you have some middleware that is in charge of authentication (usually by making additional requests against db to check if session exists and is valid), but all operations on the database are performed by backend which logs in as one user.

Let's say I have two tables - user, post and post has foreign key 'author' relating to 'username' in user table. Is it possible to have the following query on backend

SELECT * FROM post

and have it protected by RLS so the data is protected by the database itself (and have it return only posts from the user that backend is making request for)? If i connect to db as 'postgres' user, is 'current_user' postgres, or is there a way to make requests from backend as selected user. Maybe it's not the purpose of RLS, i would be glad if you could point me to some sources that could help me understand database <-> backend relation better. I feel like it is not really that complicated and my brain just didn't 'click' yet :P