Question – Visual studio related issue

Nah, you cannot mess it up while downloading or installing – that would make the install fail. Generally, VS is ripe with issues such as these. Intellisense breaking is among the most common, you should fine some tips if you search for „unity visual studio intellisense broken“.

There are of course other possible issues. What you see may also happen if you open just the script file, rather than the Visual Studio solution (.sln file in the project root). If you double click a .cs script inside the editor it should open the solution. But if you doubleclick the .cs file in explorer this will open just the script, and thus VS cannot make sense of external references.

Or generating the .sln and .csproj files may be incomplete. Check Preferences => External Tools to make sure VS is set as the script editor and try clicking „regenerate project files“.

And be sure to use the VS version bundled with Unity. This is the first item if you install Unity via the Unity Hub (recommended).