When i type OnTrigger or OnCollision i don't get completions like you do.
It seems i get completions only for objects or functions declared in the same file/project.
Yes I do use a few extensions. If you're using VS Code I would recommend going through this article first: https://code.visualstudio.com/docs/other/unity
These are the current extensions I have with VS Code:
C# for Visual Studio Code (powered by OmniSharp)
https://marketplace.visualstudio.com/items?itemName=ms-vscode.csharp
Unity Debugger Extension for Visual Studio Code
https://marketplace.visualstudio.com/items?itemName=Unity.unity-debug
Unity Tools for Visual Studio Code
https://marketplace.visualstudio.com/items?itemName=Tobiah.unity-tools
I also have the Shader Lab extension installed but haven't used it much
ShaderlabVSCodeFree
https://marketplace.visualstudio.com/items?itemName=amlovey.shaderlabvscodefree