Eglot is the built-in LSP client. You need an external server. If you do C or C++, clangd is likely what you want. Install it (apt install clangd) and enable eglot for C or C++ files in your Emacs config, (add-hook 'c-mode-hook 'eglot-ensure).
Emacs is usually batteries included, couldn't an lsp server have been part of the package..
AFAIK, the only alternative to the clangd language server is ccls: https://github.com/MaskRay/ccls