for the intellisense work with unity you need to install .Net Framework 4.7.1, .Net 5 SDK or latest and C# extension then open vscode on the root of the project to do this you have two options open directly in vscode or in unity go to assets in the top menu and click open c# project . maybe take some time to omnisharp (C# server league include on the C# extension) load work project

Vscode

Unity

If the autocomplete doesn't work try regenerate project files, also in the bottom left has a flame icon click on it and read the log and search for erros, if the button does not appear click on the dropdown and switch to visual studio or Rider, then go back to vscode