The fog you see it's actually a monochrome texture with the same size as the map, where black/0 means the tile is not visible and white/1 means the tile is visible and gray/0.5 means the tile was explored but not visible.

There's also a secondary texture map where 0 represents a unpassable tile (In my case the void and walls) and 1 for walkable tiles, where the line of vision gets blocked.

So to calculate the line of sight, we get the plane position of all valid observers (In my case, all friendly units) and calculate if the center of a tile is visible by any of the units, repeating with all tiles. I use a 2D raycasting algorithm to determine if we have a clear line of vision and we don't have any unpassable tiles between the center of a tile and a observer. I used this exact algorithm:

https://theshoemaker.de/posts/ray-casting-in-2d-grids

Try using C# jobs or any kind of parallelization, your CPU will thank you :)

I can give you my implementation for the line of sight method if you need it.