Yup, as new as they come.
Following tutorials, I'm trying to understand tilemaps (4.3) and the TileMapLayer.
All the tutorials show the viewport displaying a golden grid to represent where the tiles go. I don't get that.

I can change from square to isometric, but no change in viewport. Where is my grid?
I can zoom all the way in (over 200%) and still no grid lines.

In 4.3 Tilemaps were reworked. You only need TileMapLayers now. Create multiple of them for foreground, background etc.
What does your scene tree look like?

Is your grid enabled? Blue highlighted button: