That first link doesn’t really explain anything about syntax/autocomplete. It’s just about config options per language (formatted, linter, lsp) but I guess I was expecting something else?

It explains it in clear terms:

"""

Zed's language support is built on two main technologies:

    Tree-sitter: This handles syntax highlighting and structure-based features like the outline panel.
    Language Server Protocol (LSP): This provides semantic features such as code completion and diagnostics.

"""

Note this is _not_ how VSC's C++ Intellisense works. The VSC C++ plugin uses proprietary features of MSVC, it doesn't use LSP.