Install extension https://github.com/microsoft/vscode-arduino

Create a .vscode/settings.json file, that makes sure, that the C/C++ extension is enabled. (By default, I use the clangd extension for intellisense.)

{
    "C_Cpp.intelliSenseEngine":"default",
    "C_Cpp.autocomplete": "default",
    "C_Cpp.errorSquiggles": "enabledIfIncludesResolve"
}

And distable the clangd extension for this workspace.