"Nested" Channel Menu/List

Hey all,

I have a client who can see a down arrow for a teams channel that I built for all private channels in the team. He wants me to build a 2nd set of "nested" channels, but I have no idea how this function works and I can't find anything by google.

What my client sees:

What I see (no "nested menu")

My client says he click on the down arrow and it will hide all private channels. It will then become an up arrow and can show all private channels again. The client is asking me to make a "2nd nested menu" with another private channel.

I believe what he wants is a second group of channels that will use this menu option.

However, I can't even see this as a feature on my end. I feel a bit silly, but I have no idea how my client has this feature or how/if I can even make a "group" of channels appear/disappear with the icon.

Any advice is appreciated.