Nginx try_file directive not working as expected

I am trying to understand the Nginx config file. Here is my app directory tree:

/app
|-- abcd.html
|-- file.php
|-- index.html
|-- index.php
|-- sss
|   `-- index.html
|-- style.css
`-- thumb.png

Here is my test nginx config:

events {}
http {
        rewrite_log on;
        error_log /var/log/nginx/localhost.error_log debug;
        server {
                include mime.types;
                root /app;
                listen 8080;
                index index.html;
                location / {
                        try_files $uri/  =404;
                }
        }
}

From my understanding of the Nginx config file, when I request for localhost:8080/, according to try_files documentation, it should check for file /, then because it is a directory, it should serve index file inside. But actually, it doesn't. I also tried try_files $uri =404;. It returns a 404 page, too. But when I try try_files $uri $uri/ =404, it works and it responds with index file. Can anyone please explain what is going on?

I also tried something else that didn't work as expected. As before, in the location context, I tried try_files /sss /sss/ =404. I expected it serves me the index file inside sss directory, no matter what my requested url is. But it gets stuck in an infinite loop of requests. It responds with a 301 redirection and requests for /sss/ url. isn't it like the previous case, when I used try_files $uri $uri/ =404?