/proc directory structure shows only processes but also contains directories for threads

Question

I was studying about the internals of Linux where I came to know about how the file descriptors of processes are kept under /proc/PID/fd. I tried to poke around an Ubuntu 18.04 installation and came across something that I cannot wrap my head around.
I have a chromium-browser process (pid 6949), for which exists a thread (pid 6961) that I confirm using htop.

The image attached below shows the output of 3 commands.

1. No directory/file named 6961 exists.
2. I have a hunch that I might not be allowed to read the location, so I try my luck with sudo, directory doesn't exist.
3. I change directory to 6961 and it allows me a change!

I'm unable to understand why ls didn't list the directory named 6961 if there exists a valid location that I can cd to.
I also tried using flags -a, -h, and -l but ls simply does not list the directory /proc/6961.

What am I missing here?

akhil@hobbes:/proc$ ls /proc | grep 6961
akhil@hobbes:/proc$ sudo ls /proc | grep 6961
akhil@hobbes:/proc$ cd /proc/6961
akhil@hobbes:/proc/6961$

Answer 1

The explanation of this behavior can be found in the proc(5) manual page. Here is a shortened version of the Overview section:

   /proc/pid subdirectories
          ...

          The /proc/pid subdirectories are visible when iterating
          through /proc with getdents(2) (and thus are visible when
          one uses ls(1) to view the contents of /proc).

   /proc/tid subdirectories
          ...

          The /proc/tid subdirectories are not visible when
          iterating through /proc with getdents(2) (and thus are not
          visible when one uses ls(1) to view the contents of
          /proc).

Processes (threads whose thread IDs are equal their process IDs) can be seen under /proc using ls. Other threads are not visible using ls, but their directories can be accessed when their path is specified.