Not a lawyer and what I'm going to say is vastly oversimplified, but the gist is:

- A 501(c)(3) is for the benefit of the public

- A 501(c)(6) is for the common interests of their members

They both have advantages and disadvantages and are appropriate for certain scenarios.

Examples are:

- A 501(c)(3) Python, Scala, Zig, Mozilla, Apache, SFC, FSF

- A 501(c)(6) .NET, Rust, Eclipse