What is on the “charter web”? Is it different than the dark web? What is a closed shell system? Please explain?

I heard there is a part of the net that requires a “closed shell system” or CSS to access.

I heard rumors of bulletin board systems that are accessed via old dial up modems which connect to each other via modem lines and such. So called BBS networks are said to exist as some sort of “closed shell system”

I also heard of secret VPNs that link hacker spaces and secret websites used by hackers and such that are used by both black hat and white hat hackers

I also heard about some sort of “secret hacker social network” that is not searchable and only accessible via a closed shell network and special hack software. Access is supposedly only by invite and used by hackers to share info about exploits and to upload exe files and Vbs scripts and Js and Python files like most people upload and post pics.

These hacker sites only connect via LANs, VPNs Bluetooth, and other channels and never become accessible to the public?

Does this so called charter web exist? What parts of the whole charter web I describes are real? What is a myth? Please explain?