Flymake is only useful if at least one backend is configured to
provide the buffer-checking service. This is done via the hook
flymake-diagnostic-functions. See Hooks in The
Emacs Editor.
It’s possible that some major modes or a third-party package has
already setup this hook for you, by adding backend functions to
flymake-diagnostic-functions. If you know Elisp you may also
write your own Flymake backend functions. See Backend functions.
When the Emacs LSP support mode Eglot is enabled, Flymake will use that as an additional back-end automatically. See Eglot Features in Eglot: The Emacs LSP Client