@albertyeah I open the file? there is nothing to do for some basic highlighting.

for auto-complete and other IDE-like features Kate makes use of "LSP", plugins that talk to daemons which do the auto-complete. For C++ this would be clangd, I think its just a matter of installing it and enabling the Kate plugin.

uploaded picture is an example, it knows which fields are available in the "sum" struct.
it "just works" for cmake-based projects. for other types use "bear" to generate command list